JythonBook / IntroToPylons.rst

june...@gmail.co… 28b0486 

























































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































   1
   2
   3
   4
   5
   6
   7
   8
   9
  10
  11
  12
  13
  14
  15
  16
  17
  18
  19
  20
  21
  22
  23
  24
  25
  26
  27
  28
  29
  30
  31
  32
  33
  34
  35
  36
  37
  38
  39
  40
  41
  42
  43
  44
  45
  46
  47
  48
  49
  50
  51
  52
  53
  54
  55
  56
  57
  58
  59
  60
  61
  62
  63
  64
  65
  66
  67
  68
  69
  70
  71
  72
  73
  74
  75
  76
  77
  78
  79
  80
  81
  82
  83
  84
  85
  86
  87
  88
  89
  90
  91
  92
  93
  94
  95
  96
  97
  98
  99
 100
 101
 102
 103
 104
 105
 106
 107
 108
 109
 110
 111
 112
 113
 114
 115
 116
 117
 118
 119
 120
 121
 122
 123
 124
 125
 126
 127
 128
 129
 130
 131
 132
 133
 134
 135
 136
 137
 138
 139
 140
 141
 142
 143
 144
 145
 146
 147
 148
 149
 150
 151
 152
 153
 154
 155
 156
 157
 158
 159
 160
 161
 162
 163
 164
 165
 166
 167
 168
 169
 170
 171
 172
 173
 174
 175
 176
 177
 178
 179
 180
 181
 182
 183
 184
 185
 186
 187
 188
 189
 190
 191
 192
 193
 194
 195
 196
 197
 198
 199
 200
 201
 202
 203
 204
 205
 206
 207
 208
 209
 210
 211
 212
 213
 214
 215
 216
 217
 218
 219
 220
 221
 222
 223
 224
 225
 226
 227
 228
 229
 230
 231
 232
 233
 234
 235
 236
 237
 238
 239
 240
 241
 242
 243
 244
 245
 246
 247
 248
 249
 250
 251
 252
 253
 254
 255
 256
 257
 258
 259
 260
 261
 262
 263
 264
 265
 266
 267
 268
 269
 270
 271
 272
 273
 274
 275
 276
 277
 278
 279
 280
 281
 282
 283
 284
 285
 286
 287
 288
 289
 290
 291
 292
 293
 294
 295
 296
 297
 298
 299
 300
 301
 302
 303
 304
 305
 306
 307
 308
 309
 310
 311
 312
 313
 314
 315
 316
 317
 318
 319
 320
 321
 322
 323
 324
 325
 326
 327
 328
 329
 330
 331
 332
 333
 334
 335
 336
 337
 338
 339
 340
 341
 342
 343
 344
 345
 346
 347
 348
 349
 350
 351
 352
 353
 354
 355
 356
 357
 358
 359
 360
 361
 362
 363
 364
 365
 366
 367
 368
 369
 370
 371
 372
 373
 374
 375
 376
 377
 378
 379
 380
 381
 382
 383
 384
 385
 386
 387
 388
 389
 390
 391
 392
 393
 394
 395
 396
 397
 398
 399
 400
 401
 402
 403
 404
 405
 406
 407
 408
 409
 410
 411
 412
 413
 414
 415
 416
 417
 418
 419
 420
 421
 422
 423
 424
 425
 426
 427
 428
 429
 430
 431
 432
 433
 434
 435
 436
 437
 438
 439
 440
 441
 442
 443
 444
 445
 446
 447
 448
 449
 450
 451
 452
 453
 454
 455
 456
 457
 458
 459
 460
 461
 462
 463
 464
 465
 466
 467
 468
 469
 470
 471
 472
 473
 474
 475
 476
 477
 478
 479
 480
 481
 482
 483
 484
 485
 486
 487
 488
 489
 490
 491
 492
 493
 494
 495
 496
 497
 498
 499
 500
 501
 502
 503
 504
 505
 506
 507
 508
 509
 510
 511
 512
 513
 514
 515
 516
 517
 518
 519
 520
 521
 522
 523
 524
 525
 526
 527
 528
 529
 530
 531
 532
 533
 534
 535
 536
 537
 538
 539
 540
 541
 542
 543
 544
 545
 546
 547
 548
 549
 550
 551
 552
 553
 554
 555
 556
 557
 558
 559
 560
 561
 562
 563
 564
 565
 566
 567
 568
 569
 570
 571
 572
 573
 574
 575
 576
 577
 578
 579
 580
 581
 582
 583
 584
 585
 586
 587
 588
 589
 590
 591
 592
 593
 594
 595
 596
 597
 598
 599
 600
 601
 602
 603
 604
 605
 606
 607
 608
 609
 610
 611
 612
 613
 614
 615
 616
 617
 618
 619
 620
 621
 622
 623
 624
 625
 626
 627
 628
 629
 630
 631
 632
 633
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
Chapter 15:  Introduction to Pylons
+++++++++++++++++++++++++++++++++++

Although Django is currently the most popular webframework for Python, it is by no means your only choice.
Where Django grew out of the needs of newsrooms to implement content management solutions rapidly, Pylons grew out of a need to build web applications in environments that may have existing databases to integrate with, and the applications don’t fit neatly into the class of applications that are loosely defined in the “content management” space.

Pylons greatest strength is that it takes a best-of-breed approach to constructing its technology stack.
Where everything is “built in” with Django and the entire application stack is specifically designed with a single worldview of how applications should be done, Pylons takes precisely the opposite approach.
Pylons, the core codebase that lives in the pylons namespace, is remarkably small.
With the 0.9.7 release, it’s hovering around 5,500 lines of code.
Django, by comparison, weighs in at about 125,000 lines of code.

Pylons manages to do this magic by leveraging existing libraries extensively, and the Pylons community works with many other Python projects to develop standard APIs to promote interoperability.

Ultimately, picking Django or Pylons is about deciding which tradeoffs you’re willing to make.
Although Django is extremely easy to learn because all the documentation is in one place and all the documentation relating to any particular component is always discussed in the context of building a web application, you lose some flexibility when you need to start doing things that are at the margins of what Django was designed for.

For example, in a project we’ve worked on recently, we needed to interact with a nontrivial database that was implemented in SQL Server 2000. For Django, implementing the SQL Server back-end was quite difficult.
There aren’t that many web developers using Django on Windows, never mind SQL Server.
While the Django ORM is a part of Django, it is also not the core focus of Django.
Supporting arbitrary databases is simply not a goal for Django, and rightly so.

Pylons uses SQLAlchemy, which is probably the most powerful database toolkit available in Python.
It *None* focuses on database access.
The SQL Server back-end was already built in a robust way for CPython, and implementing the extra code for a Jython backend took two days—and this was without seeing any of the code in SQLAlchemy’s internals.

That experience alone sold us on Pylons.
We don’t have to rely on the webframework people being experts in databases.
Similarly, we don’t have to rely on the database experts to know anything about web templating.

In short, when you have to deal with the non-standard things, Pylons makes a fabulous choice, and, let’s be honest, there’s almost always non-standard things you’re going to have to do.


A Guide for the Impatient
=========================

The best way to install Pylons isinside of a virtualenv. Create a new virtualenv
for Jython and run easy_install:

*Listing 15-1.*
::
    
    > easy_install "Pylons==0.9.7"
    
Create your application:
::
    
    > paster create --template=pylons RosterTool

After initiating this command, you’ll be prompted to enter information about your application.
Let’s accept the defaults for everything by just hitting Enter at each prompt.
After you’ve entered the default information, your application will be created.
You will see a directory named the same as your application created within your current directory.
Go into that directory and you will see a series of .py files along with a couple of other files.
To configure your development environment, open up the development.ini file in a text editor.
You will see that there are several parameters in the file that can be changed, including email_to for the site administrator’s email address, smtp_server if you wish to configure mail for application, and many more.
For the purposes of this example, we’ll leave the default values in the configuration file and continue.

Next, launch the development server using the following command from within the application (*RosterTool)* directory:

::
    
    > paster serve --reload development.ini

Open a browser and connect tohttp://127.0.0.1:5000/, and you should see
something that lookssimilar to Figure 15-1.

|picture_0|

*Figure 15-1. Pylons server *

Now that we have the development server running, it is good to note that we can stop the server at any time by pressing Ctrl+C on the keyboard.

Now we have the base application created, and it is time to add some content.
It is easy to create static HTML files for your Pylons application.
Let’s create a static file named welcome.html and drop it into RosterTool/rostertool/public/ directory.

*Listing 15-2.*

::
    
        <html>
            <body>Just a static file</body>
        </html>

|picture_0|

You should now be able to load the static content by taking your browser to http://localhost:5000/welcome.html.

To create web content that handles requests, we need to create a controller for our application.
A controller is a Python module that we map to a URL so that when the URL is visited, the controller is invoked.
To add a controller, let’s start by initiating the following command:

::
    
    RosterTool/rostertool > paster controller roster

Paste will install a directory named “controllers” inside the rostertool directory and install some files in there including a module named roster.py.
You can open it up and you’ll see a class named “RosterController” and it will have a single method “index.” Pylons is smart enough to automatically map a URL to a controller classname and invoke a method.
We’ll learn more about mapping URLs later in the chapter.
To invoke the RosterController’s index method, you just need to start the development server again and invoke the following:
::
    
    http://localhost:5000/roster/index

Congratulations, you’ve got your most basic possible web application running now.
It handles basic HTTP GET requests and calls a method on a controller and a response comes out.
Let’s cover each of these pieces in detail now.


A Note about Paste
==================

While you were setting up your toy Pylons application, you probably wondered why Pylons seems to use a command line tool called “paster” instead of something obvious like “pylons.” Paster is actually a part of the Paste set of tools that Pylons uses.

Paste is used to build web applications and frameworks, but most commonly it is used to build web application frameworks like Pylons.
Every time you use “paster,” that’s Paste being called.
Every time you access the HTTP request and response objects, that’s WebOb, a descendant of Paste’s HTTP wrapper code.
Pylons uses Paste extensively for configuration management, testing, basic HTTP handling with WebOb.
You would do well to at least skim over the Paste documentation to see what is available in paste, it is available at http://pythonpaste.org/.



Pylons MVC
==========


Pylons, like Django and any reasonably sane webframework (or GUI toolkit for that matter) uses the model-view-controller design pattern.

Table 15-1 shows what this maps to in Pylons.

**Table 15-1. **Pylon MVC design pattern mapping

+------------+-------------------------------------------------------+
| Component  | Implementation                                        |
+============+=======================================================+
| Model      | SQLAlchemy (or any other database toolkit you prefer) |
+------------+-------------------------------------------------------+
| View       | Mako (or any templating language you prefer)          |
+------------+-------------------------------------------------------+
| Controller | Plain Python code                                     |
+------------+-------------------------------------------------------+



To reiterate, Pylons is about letting you, the application developer, decide on the particular tradeoffs you’re willing to make.
If using a template language more similar to the one in Django is better for your web designers, then switch go Jinja2. If you don’t really want to deal with SQLAlchemy, you can use SQLObject, files, a non-relational database, or raw SQL, if you prefer.

Pylons provides tools to help you hook these pieces together in a rational way.

**Routes** is a library that maps URLs to classes.
This is your basic mechanism for dispatching methods whenever your webserver is hit.
Routes provides similar functionality to what Django’s URL dispatcher provides.

**Webhelpers** is the defacto standard library for Pylons.
It contains commonly used functions for the web, such as flashing status messages to users, date conversion functions, HTML tag generation, pagination functions, text processing, and the list goes on.

Pylons also provides infrastructure so that you can manipulate things that are particular to web applications including:

- WSGI middleware to add cross-cutting functionality to your application with minimal intrusion into your existing codebase.

- A robust testing framework, including a shockingly good debugger you can use through the web.

- Helpers to enable REST-ful API development so you can expose your application as a programmatic interface.

Later in this chapter, we’ll wrap up the hockey roster up in a web application.
We’ll target a few features:

- Form handling and validation to add new players through the web

- Login and authentication to make sure not anybody can edit our lists

- Add a JSON/REST api so that we can modify data from other tools

In the process, we’ll use the interactive debugger from both command line and through the web to directly observe and interact with the state of the running application.


An Interlude into Java’s Memory Model
=====================================

A note about reloading: sometimes if you’re doing development with Pylons on Jython, Java will through an OutOfMemory error like this:

*Listing 15-3.*
::
    
        java.lang.OutOfMemoryError: PermGen space
            at java.lang.ClassLoader.defineClass1(Native Method)
            at java.lang.ClassLoader.defineClass(ClassLoader.java:620)

ava keeps track of class definitions in something called the Permanent Generation heap space.
This is a problem for Pylons when the HTTP threads are restarted and your classes are reloaded.
The old class definitions don’t go away; they never get garbage collected.
Because Jython is dynamically creating Java classes behind the scenes, each time your development server restarts, you’re potentially getting hundreds of new classes loaded into the JVM.

Repeat this several times and it doesn’t take long until your JVM has run out of permgen space and it keels over and dies 

To modify the permgen heap size, you’ll need to instruct Java using some extended command line options.
To set the heap to 128M, you’ll need to use “-XX:MaxPermSize=128M.”

To get this behavior by default for Jython, you’ll want to edit your Jython startup script in JYTHON_HOME/bin/jython (or jython.bat) by editing the line that reads:


*Listing 15-4.*
::
    
    set_JAVA_OPTS=

to be
::
    
    set_JAVA_OPTS=-XX:MaxPermSize=128M

This shouldn’t be a problem in production environments where you’re not generating new class definitions during runtime, but it can be quite frustrating during development.


Invoking the Pylons Shell
=========================

Yes, we’re going to start with testing right away because it will provide you with a way to explore the Pylons application in an interactive way.

Pylons gives you an interactive shell much like Django’s. You can start it up with the following commands:

*Listing 15-5.*
::
    
    RosterTool > jython setup.py egg_info
    RosterTool > paster shell test.ini

This will yield a nice interactive shell you can start playing with right away.
Now let’s take a look at those request and response objects in our toy application.


*Listing 15-6.*
::
    
    RosterTool > paster shell test.ini
    
    Pylons Interactive Shell
    Jython 2.5.0 (Release_2_5_0:6476, Jun 16 2009, 13:33:26)
    [OpenJDK Server VM (Sun Microsystems Inc.)]
    
    All objects from rostertool.lib.base are available
    Additional Objects:
    mapper     -  Routes mapper object
    wsgiapp    -  This project's WSGI App instance
    app        -  paste.fixture wrapped around wsgiapp
    
    >>> resp = app.get('/roster/index')
    >>> resp
    <Response 200 OK 'Hello World'>
    >>> resp.req
    <Request at 0x43 GET http://localhost/roster/index>


Pylons lets you actually run requests against the application and play with the resulting response.
Even for something as “simple” as the HTTP request and response, Pylons uses a library to provide convenience methods and attributes to make your development life easier.
In this case, it’s WebOb.

The request and the response objects both have literally dozens of attributes and methods that are provided by the framework.
You will almost certainly benefit if you take time to browse through WebOb’s documentation, which is available at http://pythonpaste.org/webob/.

Here’s four attributes you really have to know to make sense of the request object.
The best thing to do is to try playing with the request object in the shell.

request.GET
-----------

GET is a special dictionary of the variables that were passed in the URL.
Pylons automatically converts URL arguments that appear multiple times into discrete key value pairs.

*Listing 15-7.*
::
    
    >>> resp = app.get('/roster/index?foo=bar&x=42&x=50')
    >>> resp.req.GET
    UnicodeMultiDict([('foo', u'bar'), ('x', u'42'), ('x', u'50')])
    >>> resp.req.GET['x']
    u'50'
    >>> resp.req.GET.getall('x')
    [u'42', u'50']


Note how you can get either the last value or the list of values depending on how you choose to fetch values from the dictionary.
This can cause subtle bugs if you’re not paying attention.


request.POST
------------

POST is similar to GET, but appropriately: it only returns the variables that were sent up during an HTTP POST submission.

request.params
--------------

Pylons merges all the GET and POST data into a single MultiValueDict.
In almost all cases, this is the one attribute that you really want to use to get the data that the user sent to the server.

request.headers
---------------

This dictionary provides all theHTTP headers that the client sent to the server.


Context Variables and Application Globals
=========================================

Most web frameworks provide a request scoped variable to act as a bag of values.
Pylons is no exception: whenever you create a new controller with paste, it will automatically import an attribute ‘c’ which is the context variable.

This is one aspect of Pylons that we’ve found to be frustrating.
The ‘c’ attribute is code generated as an import when you instruct paste to build you a new controller.
The ‘c’ value is *None* an attribute of your controller—Pylons has special global threadsafe variables, this is just one of them.
You can store variables that you want to exist for the duration of the request in the context.
These values won’t persist after the request/response cycle has completed, so don’t confuse this with the session variable.

The other global variable you’ll end up using a lot is pylons.session.
This is where you’ll store variables that need to persist over the course of several request/response cycles.
You can treat this variable as a special dictionary: just use standard Jython dictionary syntax and Pylons will handle the rest.

Routes
======

Routes is much like Django’s URL dispatcher.
It provides a mechanism for you to map URLs to controllers classes and methods to invoke.

Generally, we find that Routes makes a tradeoff of less URL matching expressiveness in exchange for simpler reasoning about which URLs are directed to a particular controller and method.
Routes doesn’t support regular expressions, just simple variable substitution.

A typical route will look something like this

map.connect('/{mycontroller}/{someaction}/{var1}/{var2}')

This route would find the controller called “mycontroller” (note the casing of the class) and invoke the “someaction” method on that object.
Variables var1 and var2 would be passed in as arguments.

The connect() method of the map object will also take in optional arguments to fill in default values for URLs that do not have enough URL-encoded data in them to properly invoke a method with the minimum required number of arguments.
The front page is an example of this; let’s try connecting the frontpage to the Roster.index method.

Edit RosterTool/rostertool/config/routing.py so that there are 3 lines after #CUSTOM_ROUTES_HERE that should read like the following

*Listing 15-8.*
::
    
    map.connect('/', controller='roster',action='index')  ---
    map.connect('/{action}/{id}/',controller='roster')  ---
    map.connect('/add_player/',controller='roster', action='add_player')

While this *None* like it should work, you can try running paster server: it won’t.

By default, Pylons always tries to serve static content before searching for controllers and methods to invoke.
You’ll need to go to RosterTool/rostertool/public and delete the “index.html” file that paster installed when you first created your application.
If you wanted to change the default implementation, you could tweak the middleware*.py* module to your liking.

Load http://localhost:5000/ again in your browser—the default index.html should be gone and you should now get your response from the controller method that is mapped to index, in this case, you should see “Hello World.”

Controllers and Templates
=========================

Leveraging off of the Table model we defined in Chapter 12, let’s create the hockey roster, but this time using the PostgreSQL database.
We’ll assume that you have a PostgreSQL installation running that allows you to create new databases.
You can also use a different database if you choose by simply creating a different engine with SQLAlchemy.
For more details, please visit the documentation about creating database engines at http://www.sqlalchemy.org/docs/05/dbengine.html.

Begin by opening up a Pylons interactive shell and typing the following commands:

*Listing 15-9.*
::
    
    >>> from sqlalchemy import *
    >>> from sqlalchemy.schema import Sequence
    >>> db = create_engine('postgresql+zxjdbc://myuser:mypass@localhost:5432/mydb')
    >>> connection = db.connect()
    >>> metadata = MetaData()
    >>> player = Table('player', metadata,
    ...     Column('id', Integer, primary_key=True),
    ...     Column('first', String(50)),
    ...     Column('last', String(50)),
    ...     Column('position', String(30)))
    >>> metadata.create_all(engine)
Now let’s wire the data up to the controllers, display some data, and get basic form handling working.
We’re going to create a basic CRUD (create, read, update, delete) interface to the sqlalchemy model.
Because of space constraints, this HTML is going to be very basic; but you’ll get a taste of how things fit together.

Paste doesn’t just generate a stub for your controller—it will also code generate an empty functional test case in rostertool/tests/functional/ as test_roster.py.
We’ll visit testing shortly.

Controllers are really where the action occurs in Pylons.
This is where your application will take data from the database and prepare it for a template to render it as HTML.
Let's put the list of all players on the front page of the site.
We’ll implement a template to render the list of all players.
Then, we’ll implement a method in the controller to override the index() method of Roster to use SQLAlchemy to load the records from disk and send them to the template.

Along the way, we’ll touch on template inheritance so that you can see how you can save keystrokes by subclassing your templates in Mako.

First, let’s create two templates, base.html and list_players.html in the rostertool/templates directory.


*Listing 15-10. base.html*
::
    
    <html>
        <body>
            <div class="header">
                ${self.header()}
            </div>
    
            ${self.body()}
        </body>
    </html>
    
    <%def name="header()">
        <h1>${c.page_title}</h1>
        <% messages = h.flash.pop_messages() %>
        % if messages:
        <ul id="flash-messages">
            % for message in messages:
            <li>${message}</li>
            % endfor
        </ul>
        % endif
    </%def>


*Listing 15-11. list_players.html*
::
    
    <%inherit file="base.html" />
    <table border="1">
        <tr>
            <th>Position</th><th>Last name</th><th>First name</th><th>Edit</th>
        </tr>
        % for player in c.players:
            ${makerow(player)}
        % endfor
    </table>
    
    <h2>Add a new player</h2>
    ${h.form(h.url_for(controller='roster', action='add_player'), method='POST')}
        ${h.text('first', 'First Name')} <br />
        ${h.text('last', 'Last Name')} <br />
        ${h.text('position', 'Position')} <br />
        ${h.submit('add_player', "Add Player")}
    ${h.end_form()}
    
    <%def name="makerow(row)">
    <tr>
        <td>${row.position}</td>\
        <td>${row.last}</td>\
        <td>${row.first}</td>\
        <td><a href="${h.url_for(controller='roster', action='edit_player', id=row.id)}">Edit</a></td>\
    </tr>
    </%def>

There’s quite a bit going on here.
The base template lets Mako define a boilerplate set of HTML that all pages can reuse.
Each section is defined with a <%def name="block()"> section, and the blocks are overloaded in the subclassed templates.
In effect, Mako lets your page templates look like objects with methods that can render subsections of your pages.

The list_players.html template has content that is immediately substituted into the self.body() method of the base template.
The first part of our body uses our magic context variable ‘c’. Here, we’re iterating over each of the players in the database and rendering them into a table as a row.
Note here that we can use the Mako method syntax to create a method called “makerow” and invoke it directly within our template.

.. note::
    
    Mako provides a rich set of functions for templating.
    We’re only going to use the most basic parts of Mako: inheritance, variable substitution, and loop iteration to get the toy application working.
    I strongly suggest you dive into the Mako documentation to discover features and get a better understanding of how to use the template library.

Next, we add in a small form to create new players.
The trick here is to see that the form is being generated programmatically by helper functions.
Pylons automatically imports YOURPROJECT/lib/helpers (in our case, rostertool.lib.helpers) as the ‘h’ variable in your template.
The helpers module typically imports functions from parts of Pylons or a dependent library to allow access to those features from anywhere in the application.
Although this seems like a violation of “separation of concerns,” look at the template and see what it buys us: we get fully decoupled URLs from the particular controller and method that need to be invoked.
The template uses a special routes function “url_for” to compute the URL that would have been mapped for a particular controller and method.
The last part of our base.html file contains code to display alert messages.

Let’s take a look at our rostertool.lib.helpers module now.

*Listing 15-12.*
::
    
    from routes import url_for
    from webhelpers.html.tags import *
    from webhelpers.pylonslib import Flash as _Flash
    
    # Send alert messages back to the user
    flash = _Flash()

Here, we’re importing the url_for function from routes to do our URL reversal computations.
We import HTML tag generators from the main html.tags helper modules and we import Flash to provide alert messages for our pages.
We’ll show you how flash messages are used when we cover the controller code in more detail in the next couple of pages.

Now, create a controller with paste (you’ve already done this if you were impatient at the beginning of the chapter).

.. note::

    If you have already created the controller using thequickstart at the
    beginning of the chapter, you will need to add theSQLAlchemy configuration to
    the development*.ini* file by addingthe following line to the file.
    ::
        
        sqlalchemy.url= postgresql+zxjdbc://dbuser:dbpassword@dbhost:port/dbname

*Listing 15-13.*
::
    
    $ cd ROSTERTOOL/rostertool
    $ paster controller roster

Next, we need to add the metadata for our databasetable to the
RosterTool/rostertool/model/__init__.py module. To doso, change the file so that
it reads as follows:

*Listing 15-14.*
::
    
    """The application's model objects"""
    import sqlalchemy as sa
    from sqlalchemy import orm, schema, types
    
    from rostertool.model import meta
    
    def init_model(engine):
        """Call me before using any of the tables or classes in the model"""
        ## Reflected tables must be defined and mapped here
        #global reflected_table
        #reflected_table = sa.Table("Reflected", meta.metadata, autoload=True,
        #                           autoload_with=engine)
        #orm.mapper(Reflected, reflected_table)
        #
        meta.Session.configure(bind=engine)
        meta.engine = engine
    
    metadata = schema.MetaData()
    
    # Create the metadata for the player table, and assign it to player_table
    player_table = schema.Table('player', metadata,
        schema.Column('id', types.Integer, primary_key=True),
        schema.Column('first', types.Text(), nullable=False),
        schema.Column('last', types.Text(), nullable=False),
        schema.Column('position', types.Text(), nullable=False),
    )
    
    # Create a class to be used for mapping the player_table object
    class Player(object):
        pass
     
    # Map the Player class to the player_table object, we can now refer to the
    # player_table using Player
    orm.mapper(Player, player_table)

Note that we are creatingthe proper metadata for mapping to the player database
table. We thencreate an empty Player class object andlater use the
orm*.*mapper**to map the metadata to the emptyPlayer object. We can now use the
Player object to work with ourdatabase table.

Next, we should alter the index method that iscreated inside the RosterContoller
class. We will add an import tobring in the meta**and Player**objects, and
change the index**function so that it queriesthe list of players in the
database. In the end, the index**function should read as follows:

*Listing 15-15.*
::
    
    from rostertool.model import meta, Player
    ...
    def index(self):
        session = meta.Session()
        c.page_title = 'Player List'
        c.players = session.query(Player).all()
        return render('list_players.html')

This code is fairly straightforward; we are simplyusing a SQLAlchemy session to
load all the Player objects from diskand assigning to the special context
variable c. Pylons is theninstructed to render the list_player.html file.

The context should be your default place to placevalues you want to pass to
other parts of the application. Note thatPylons will automatically bind in URL
values to the context so whileyou can grab the form values from
self.form_result, you can also grabraw URL values from the context.

You should be able run the debug webserver now andyou can get to the front page
to load an empty list of players. Startup your debug webserver as you did at the
beginning of this chapterand go to http://localhost:5000/ to see the page load
with your listof players (currently an empty list).

Now we need to get to the meaty part where we canstart create, edit, and delete
players. Well make sure that theinputs are at least minimally validated, errors
are displayed to theuser, and that alert messages are properly populated.

First, we need a page that shows just a singleplayer and provides buttons for
edit and delete.

*Listing 15-16.*
::
    
    <%inherit file="base.html" />
    
    <h2>Edit player</h2>
    ${h.form(h.url_for(controller='roster', action='save_player', id=c.player.id), method='POST')}
        ${h.hidden('id', c.player.id)} <br />
        ${h.text('first', c.player.first)} <br />
        ${h.text('last', c.player.last)} <br />
        ${h.text('position', c.player.position)} <br />
        ${h.submit('save_player', "Save Player")}
    ${h.end_form()}
    
    ${h.form(h.url_for(controller='roster', action='delete_player', id=c.player.id), method='POST')}
        ${h.hidden('id', c.player.id)} <br />
        ${h.hidden('first', c.player.first)} <br />
        ${h.hidden('last', c.player.last)} <br />
        ${h.hidden('position', c.player.position)} <br />
        ${h.submit('delete_player', "Delete Player")}
    ${h.end_form()}


This template assumes that there is a playervalue assigned to the context and,
not surprisingly, its going tobe a full blown instance of the Player object that
we first saw inChapter 12. The helper functions let us define our HTML form
usingwebhelper tag generation functions. This means you wont have toworry about
escaping characters or remembering the particular detailsof the HTML attributes.
The helper.tag functions will do sensiblethings by default. The *h* is a default
template variable thatrefers to the repository of helper functions.

Weve set up the edit and delete forms to pointto different URLs. You might want
to conserve URLs, but havingdiscrete URLs for each action has advantages,
especially fordebugging. You can trivially view which URLs are being hit on
awebserver by reading log files. Seeing the same kind of behavior ifthe URLs are
the same, but the behavior is dictated by some formvaluewell, thats a whole lot
harder to debug. Its also a lotharder to setup in your controllers because you
need to dispatch thebehavior on a per method level. Why not just have separate
methodsfor separate behavior? Everybody will thank you for it when they needto
debug your code in the future.

Before we create our controller methods forcreate, edit and delete, well create
a formencode schema toprovide basic validation. Again, Pylons doesnt provide
validationbehaviorit just leverages another library to do so. Add thefollowing
class to rostertool/controllers/roster.py:

*Listing 15-17.*
::
    
    class PlayerForm(formencode.Schema):
        # You need the next line to drop the submit button values
        allow_extra_fields=True
    
        first = formencode.validators.String(not_empty=True)
        last = formencode.validators.String(not_empty=True)
        position = formencode.validators.String(not_empty=True)

This simply provides basic string verification onour inputs. Note how this
doesnt provide any hint as to what theHTML form looks likeor that its HTML at
all. FormEncode canvalidate arbitrary Python dictionaries and return errors
about them.

Were just going to show you the add method, andthe edit_player methods. You
should try to implement the save_playerand delete_player methods to make sure
you understand whats goingon here. First, add the import for the
validate**decorator.Next, add the add*_*player**and edit_player functions tothe
RosterConroller class.

*Listing 15-18.*
::
    
    from pylons.decorators import validate
    
    
    @validate(schema=PlayerForm(), form='index', post_only=False, on_get=True)
    def add_player(self):
        first = self.form_result['first']
        last = self.form_result['last']
        position = self.form_result['position']
        session = meta.Session()
        if session.query(Player).filter_by(first=first, last=last).count() > 0:
            h.flash("Player already exists!")
            return h.redirect_to(controller='roster')
        player = Player(first, last, position)
        session.add(player)
        session.commit()
        return h.redirect_to(controller='roster', action='index')
    
    def edit_player(self, id):
        session = meta.Session()
        player = session.query(Player).filter_by(id=id).one()
        c.player = player
        return render('edit_player.html')

A couple of notes here. The edit_player functionis passed the id attribute
directly by Routes. In theedit_player method, player is assigned to the context,
but thecontext is never explicitly passed into the template renderer. Pylonsis
going to automatically take the attributes bound to the contextand write them
into the template and render the HTML output. The *c*variable is automatically
available in the template namespace muchlike the *h* variable as discussed
previously.

With the add_player method, were using thevalidate decorator to enforce the
inputs against the PlayerForm. Inthe case of error, the form attribute of the
decorator is used toload an action against the current controller. In this case,
index,so the front page loads.

The SQLAlchemy code should be familiar to you ifyou have already gone through
Chapter 12. The last line of theadd_player method is a redirect to prevent
problems with hittingreload in the browser. Once all data manipulation has
occurred, theserver redirects the client to a results page. In the case that
auser hits reload on the result page, no data will be mutated.

Heres the signatures of the remaining methodsyoull need to implement to make
things work:

* save_player(self):

* delete_player(self):

If you get stuck, you can always consult theworking sample code on the books web
site.


Adding a JSON API
=================

JSON integration into Pylons isvery straight forward. The steps are roughly the
same as addingcontroller methods for plain HTML views. You invoke paste, paste
thengenerates your controller stubs and test stubs, you add in someroutes to
wire controllers to URLs and then you just fill in thecontroller code.

*Listing 15-19.*
::
    
    $ cd ROSTERTOOL_HOME/rostertool
    $ paster controller api

Pylons provides a special @jsonify decorator whichwill automatically convert
Python primitive types into JSON objects.It will *not*convert the POST data into
an object though; thats yourresponsibility. Adding a simple read interface into
the player listrequires only adding a single method to your ApiController:

*Listing 15-20.*
::
    
    @jsonify
    def players(self):
        session = Session()
        players = [{'first': p.first,
                    'last': p.last,
                    'position': p.position,
                    'id': p.id} for p in session.query(Player).all()]
        return players

Adding a hook so that people can POST data to yourserver in JSON format to
create new player is almost as easy.

*Listing 15-21.*
::

import simplejson as json
    
    @jsonify
    def add_player(self):
        obj = json.loads(request.body)
        schema = PlayerForm()
        try:
            form_result = schema.to_python(obj)
        except formencode.Invalid, error:
            response.content_type = 'text/plain'
            return 'Invalid: '+unicode(error)
        else:
            session = Session()
            first, last, position = obj['first'], obj['last'], obj['position']
            if session.query(Player).filter_by(last=last, first=first,
                    position=position).count() == 0:
                session.add(Player(first, last, position))
                session.commit()
                return {'result': 'OK'}
            else:
                return {'result':'fail', 'msg': 'Player already exists'}


Unit Testing, Functional Testing, and Logging
=============================================

One of our favorite features inPylons is its rich set of tools for testing and
debugging. It evenmanages to take social networking, turn it upside-down, and
make itinto a debugger feature. Well get to that shortly.

The first step to knowing how to test code inPylons is to familiarize yourself
with the nose testing framework.Nose makes testing simple by getting out of your
way. There are noclasses to subclass, just start writing functions that start
with theword test and nose will run them. Write a class that has testprefixed in
the name and nose will treat it as a suite of testsrunning each method that
starts with test. For each testmethod, nose will execute the setup() method just
prior to executingyour test and nose will execute the teardown() method after
your testcase.

Best of all, nose will automatically hunt downanything that looks like a test
and will run it for you. There is nocomplicated chain of test cases you need to
organize in a tree. Thecomputer will do that for you.

Lets take a look at your first test case: welljust instrument the model, in this
caseSQLAlchemy. Because themodel layer has no dependency on Pylons, this
effectively tests onlyyour SQLAlchemy model code.

In ROSTERTOOL_HOME/rostertool/tests, create amodule called test_models.py with
the following content

*Listing 15-22.*
::
    
    from rostertool.model import Player, Session, engine
    
    class TestModels(object):
    
        def setup(self):
            self.cleanup()
    
        def teardown(self):
            self.cleanup()
    
        def cleanup(self):
            session = Session()
            for player in session.query(Player):
                session.delete(player)
            session.commit()
    
        def test_create_player(self):
            session = Session()
            player1 = Player('Josh', 'Juneau', 'forward')
            player2 = Player('Jim', 'Baker', 'forward')
            session.add(player1)
            session.add(player2)
    
            # But 2 are in the session, but not in the database
            assert 2 == session.query(Player).count()
            assert 0 == engine.execute("select count(id) from player").fetchone()[0]
            session.commit()
    
            # Check that 2 records are all in the database
            assert 2 == session.query(Player).count()
            assert 2 == engine.execute("select count(id) from player").fetchone()[0]

Before we can run the tests, well need to editthe model module a little so that
the models know to lookup theconnection URL from Pylons configuration file. In
your test.ini,add a line setting the sqlalchemy.url setting to point to
yourdatabase in the [app:main] section.

You should have a line that looks something likethis:

*Listing 15-23.*
::
    
     [app:main]
    use = config:development.ini
    sqlalchemy.url = postgresql+zxjdbc://username:password@localhost:5432/mydb

Now edit the model file so that the create_enginecall uses that configuration.
This is as simple as importing configfrom pylons and doing a dictionary lookup.
The two lines you want are

*Listing 15-24.*
::
    
    from pylons import config
    engine = create_engine(config['sqlalchemy.url'])

and thats it. Your model willnow lookup your database connection string from
Pylons. Even better,nose will know how to use that configuration as well.

From the command line, you can run the tests fromROSTERTOOL_HOME like this now:

*Listing 15-25.*
::
    
    ROSTERTOOL_HOME $ nosetests rostertool/tests/test_models.py
    .
    ----------------------------------------------------------------------
    Ran 1 test in 0.502s

Perfect! To capture stdout and get verbose output,you can choose to use the -sv
option. Another nice option is-pdb-failures, which will drop you into the
debugger on failures.Nose has its own active community of developers. You can
get plug-insto do coverage analysis and performance profiling with some of
theplugins. Use nosetests --help for a list of the optionsavailable for a
complete list.

Due to the nature of Pylons and its decoupleddesign, writing small unit tests to
test each little piece of code isvery easy. Feel free to assemble your tests any
which way you want.Just want to have a bunch of test functions? Great! If you
need tohave setup and teardown and writing a test class makes sense, then doso.

Testing with nose is a joyyou arent forcedto fit into any particular structure
with respect to where you testsmust go so that they will be executed. You can
organize your tests ina way that makes the most sense to *you*.

That covers basic unit testing, but suppose wewant to test the JSON interface to
our hockey roster. We really wantto be able to invoke GET and POST on the URLs
to make sure that URLrouting is working as we expect. We want to make sure that
thecontent-type is properly set to application/x-json. In otherwords, we want to
have a proper functional test, a test thats notas fine grained as a unit test.

The prior exposure to the app object when weran the paste shell should give you
a rough idea of what is required.In Pylons, you can instrument your application
code by using aTestController. Lucky for you, Pylons has already created one for
youin your :math:`<`\ app\ :math:`>`/tests directory. Just import it, subclass
it andyou can start using the app object just like you did inside ofthe shell.

Lets take a look at a functional test in detailnow. Heres a sample you can save
intorostertool/tests/functional/test_api.py:

*Listing 15-26.*
::
    
    from rostertool.tests import *
    import simplejson as json
    from rostertool.model.models import Session, Player
    
    class TestApiController(TestController):
        # Note that we're using subclasses of unittest.TestCase so we need
        # to be careful with setup/teardown camelcasing unlike nose's
        # default behavior
    
        def setUp(self):
            session = Session()
            for player in session.query(Player):
                session.delete(player)
            session.commit()
    
        def test_add_player(self):
            data = json.dumps({'first': 'Victor',
                'last': 'Ng',
                'position': 'Goalie'})
            # Note that the content-type is set in the headers to make
            # sure that paste.test doesn't URL encode our data
            response = self.app.post(url(controller='api', action='add_player'),
                params=data,
                headers={'content-type': 'application/x-json'})
            obj = json.loads(response.body)
            assert obj['result'] == 'OK'
    
            # Do it again and fail
            response = self.app.post(url(controller='api', action='add_player'),
                params=data,
                headers={'content-type': 'application/x-json'})
            obj = json.loads(response.body)
            assert obj['result'] <> 'OK'

Theres a minor detail which you can easily misswhen youre using the
TestController as your superclass. First off,TestController is a descendant of
unittest.TestCase from the standardpython unit test library. Nose will not run
setup andteardown methods on TestCase subclasses. Instead, youll haveto use the
camel case names that TestCase uses.

Reading through the testcase should show you howmuch detail you can be exposed.
All your headers are exposed, theresponse content is exposed; indeed, the HTTP
response is completelyexposed as an object for you to inspect and verify.

So great, now we can run small unit tests, biggerfunctional tests; lets take a
look at the debugging facilitiesprovided through the web.

Consider what happens with most web applicationstacks when an error occurs.
Maybe you get a stack trace, maybe youdont. If youre lucky, you can see the
local variables at eachstack frame like Django does. Usually though, youre out
of luck ifyou want to interact with the live application as the error
isoccurring.

Eventually, you may locate the part of the stacktrace that triggered the error,
but the only way of sharing thatinformation is through either the mailing lists
or by doing a formalpatch against source control. Lets take a look at an example
ofthat.

Were going to startup our application indevelopment mode. Were also going to
intentionally break some codein the controller to see the stack trace. But
first, well need toput some data into our app.

Add a sqlalchemy.url configuration line to thedevelopment.ini file as you did in
the test.ini configuration, andlets startup the application in development mode.
Were going tohave the server run so that any code changes on the file system
areautomatically detected and the code is reloaded

$paster serve development.ini --reload

Well add a single player John Doe as acenter, and save the record. Next, lets
intentionally break somecode to trigger the debugger. Modify the
RosterControllers indexmethod and edit the call that loads the list of players.
Well usethe web session instead of the database session to try loading thePlayer
objects.

*Listing 15-27.*
::
    
    def index(self):
        db_session = meta.Session()
        c.page_title = 'Player List'
        c.players = session.query(Player).all()
        return render('list_players.html')

Load http://localhost:5000/ to see the error page.

Theres a lot of information that Pylons throwsback at you. Along the top of the
screen, youll see four tabs:Traceback, Extra Data, Template, and Source. Pylons
will have put youin the Traceback tab by default to start with. If you look at
theerror, youll see the exact line number in the source file that theerror
occurred in. Whats special about Pylons traceback tab isthat this is actually a
fully interactive session.

You can select the+ signs to expand each stackframe and a text input along
withsome local variables on that frame will be revealed. That text inputis an
interface into your server process. You can type virtually anyPython command
into it, hit Enter, and you will get back liveresults. From here, we can see
that we should have used thedb_session and not the session variable. See Figure
15-2.

|picture_1|

*Figure 15-2. Error message caused by use of Session*

This is pretty fantastic. If you click on the Viewlink, you can even jump to the
full source listing of the Jythonmodule that caused the error. One bug in Pylons
at the time ofwriting is that sometimes the hyperlink is malformed. So,
althoughthe traceback will correctly list the line number that the erroroccurred
at, the source listing may go to the wrong line.

The Pylons developers have also embedded aninterface into search engines to see
if your error has beenpreviously reported. If you scroll down to the bottom of
yourtraceback page, youll see another tab control with a Search MailLists
option. Here, Pylons will automatically extract the exceptionmessage and provide
you an interface so you can literally search allthe mailing lists that are
relevant to your particular Pylonsinstallation.

If you cant find your error on the mailinglists, you can go to the next tab,
Post Traceback, and submit yourstacktrace to a webservice on PylonsHQ.com so
that you can try todebug your problems online with other collaborators.
Combining unittests, functional tests, and the myriad of debugging options
affordedto you in the web debugger, Pylons makes the debugging experience
aspainless as possible.


Deployment into a Servlet Container
===================================

Deploying your pylons applicationinto a servlet container is very straight
forward. Just installsnakefight from PyPI using easy_install and you can start
buildingWAR files.

*Listing 15-28.*
::
    
    $ easy_install snakefight
    ...snakefight will download and install here ...
    $ jython setup.py bdist_war --paste-config test.ini

By default, snakefight will bundle a completeinstance of your Jython
installation into the WAR file. What itdoesnt include is any JAR files that your
application depends on.For our small example, this is just the postgresql JDBC
driver. Youcan use the --include-jars options and provide a comma separated
listof JAR files.

*Listing 15-29.*
::
    
    $ jython setup.py bdist_war \
        --include-jars=postgresql-8.3-604.jdbc4.jar \
        --paste-config=test.ini

The final WAR file will be located under the distdirectory. It will contain your
postgreql JDBC driver, a completeinstallation of Jython including anything
located in site-packagesand your application. Your war file should deploy
without any issuesinto any standards compliant servlet container.


Summary
=======

Weve only scratched the surfaceof whats possible, but I hope youve gotten a
taste of what ispossible with Pylons. Pylons uses a large number of packages so
youwill need to spend more time getting over the initial learning curve,but the
dividend is the ability to pick and choose the libraries thatbest solve your
particular problems. It would be helpful to take alook at some other resources
such as *The Definitive Guide toPylons* from Apress, which is also available
online athttp://pylonsbook.com.

.. |picture_1| image:: images/picture_1.png

.. |picture_0| image:: images/picture_0.png
Tip: Filter by directory path e.g. /media app.js to search for public/media/app.js.
Tip: Use camelCasing e.g. ProjME to search for ProjectModifiedEvent.java.
Tip: Filter by extension type e.g. /repo .js to search for all .js files in the /repo directory.
Tip: Separate your search with spaces e.g. /ssh pom.xml to search for src/ssh/pom.xml.
Tip: Use ↑ and ↓ arrow keys to navigate and return to view the file.
Tip: You can also navigate files with Ctrl+j (next) and Ctrl+k (previous) and view the file with Ctrl+o.
Tip: You can also navigate files with Alt+j (next) and Alt+k (previous) and view the file with Alt+o.