baker /

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Version 1.1
  • now prints the return value of the command function.
  • Command usage help now shows help for optional arguments.
  • Added options to
  • Added baker.usage([commandname]).
  • Added unit tests.
  • Fixed bugs.


Baker lets you easily add a command line interface to your Python functions using a simple decorator, to create scripts with "sub-commands", similar to Django's, svn, hg, etc.:

import baker

# An imaginary script full of useful Python functions

def set(name, value=None, overwrite=False):
        """Sets the value of a key in the database.

        If you don't specify a value, the named key is deleted. Overwriting
        a value may not be visible to all clients until the next full sync.

    db = get_database()
    if overwrite or name not in db:
        if value is None:
                print "Deleted %s" % name
                db.set(name, value)
                print "Set %s to %s" % (name, value)
        print "Key exists!"

def get(name):
        "Prints the value of a key in the database."

        db = get_database()
        print db.get(name)

You can then run the script and use your function names and parameters as the command line interface, using optparse-style options:

$ set alfa bravo
Set alfa to bravo

$ set --overwrite alfa charlie
Set alfa to charlie

$ get alfa

$ --help

Available commands:

 get  Prints the value of a key in the database.
 set  Sets the value of a key in the database

Use " <command> --help" for individual command help.

$ set --help

Usage: set <name> [<value>]

Sets the value of a key in the database.

    If you don't specify a value, the named key is deleted. Overwriting
        a value may not be visible to all clients until the next full sync.




Baker maps command line options to function parameters in the most natural way available.

Bare arguments are used to fill in required parameters:

def test(a, b, c):
  print "a=", a, "b=", b, "c=", c

$ test 1 2 3
a= 1 b= 2 c= 3

--option arguments are used to fill in keyword parameters. You can use --option value or --option=value, as in optparse:

def test(key="C"):
        print "In the key of:", key

$ test
In the key of: C
$ test --key A
In the key of: A
$ test --key=Gb
In the key of: Gb

Function parameters where the default is None are considered optional arguments and will be filled if extra arguments are available. Otherwise, extra bare arguments never fill in keyword parameters:

def test(start, end=None, sortby="time"):
  print "start=", start, "end=", end, "sort=", sortby

$ --sortby name 1
start= 1 end= sortby= name
$ 1 2
start= 1 end= 2 sortby= time

If a keyword parameter's default is an int or float, Baker will try to convert the option's string to the same type:

def test(limit=10):
        print type(limit)

$ test --limit 10
<type 'int'>

If the default of a parameter is a boolean, the corresponding command line option is a flag that sets the opposite of the default:

def test(name, verbose=False):
  if verbose: print "Opening", name

$ test --verbose alfa
Opening alfa

If the function takes * and/or ** parameters, any leftover arguments and options will fill them in.

Parameter help

Baker lets you specify help for parameters in three ways.

In the decorator:

@baker.command(params={"force": "Delete even if the file exists"})
def delete(filename, force=False):
        "Deletes a file."
        if force or not os.path.exists(filename):

In Python 3.x, you can use parameter annotations to associate doc strings with parameters:

def delete(filename, force:"Delete even if the file exists."=False):
    "Deletes a file."
            if force or not os.path.exists(filename):

Baker can parse the function's docstring for Sphinx-style :param blocks:

def delete(filename, force=False):
        """Deletes a file.

        :param force: Delete even if the file exists.
        if force or not os.path.exists(filename):

Short options

To allow single-character short options (e.g. -v for --verbose), use the shortopts keyword on the decorator:

@baker.command(shortopts={"verbose": "v"}, params={"verbose", "Spew lots"})
def test(verbose=False):

$ test --help

Usage: test


 -v --verbose  Spew lots

You can group multiple short flag options together (-xvc). You can also optionally not put a space between a short option and its argument, for example -nCASE instead of -n CASE.

run() function

The run() function has a few useful options.

  • argv: the list of options to parse. Default is sys.argv.
  • main: if True (the default), this function acts like the main function of the module -- it prints errors instead of raising exceptions, prints the return value of the command function, and exits with an error code on errors.
  • help_on_error: if True, when an error occurs, automatically prints the usage help after the error message. Default is False.
  • outfile, errorfile, helpfile: the files to use for output, errors, and usage help. Defaults are stdout, stderr, and stdout.
  • errorcode: if main=True and this value is not 0, calls sys.exit() with this code in the event of an error

usage() function

Use the usage() function if you need to print the usage help programmatically:

# Print overall help

# Print help for a command

# Print to a file
baker.usage("commandname", file=sys.stdout)


Instead of, you can use baker.test() to print out how Baker will call your function based on the given command line.

As in many UNIX command line utilities, if you specify a single hyphen (-) as a bare argument, any subsequent arguments will not parsed as options, even if they start with --.

Commands are automatically given the same name as the decorated function. To give a command a different name, use the name keyword on the decorator. This is especially useful when the command name you want isn't a valid Python identifier:

def trackall():

You can specify a "default" command that is used when the first argument to the script doesn't look like a command name:

def here(back=False):
  print "here! back=", back

def there(back=False):
  print "there! back=", back

$ --back
here! back= True

The baker module contains a Baker class you can instantiate if you don't want to use the global functions:

mybaker = baker.Baker()

def test():
        print "hello"

About Baker

Created by Matt Chaput.

Released under the Apache 2.0 license

Please file bugs in the BitBucket issue tracker.

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