1. Drake Guan
  2. Python 3 Patterns & Idioms


Python 3 Patterns & Idioms / html / _sources / Comprehensions.txt

.. index::
   pair: list; comprehension
   pair: generator; comprehension


History: where did they come from?

They require a mind shift.

What makes them so compelling (once you 'get it')?


Using ``zip()`` and dealing with two or more elements at a time::

    ['%s=%s' % (n, v) for n, v in zip(self.all_names, self)]

Multiple types (auto unpacking of a tuple)::

    [f(v) for (n, f), v in zip(cls.all_slots, values)]

A two-level list comprehension using ``os.walk()``::

    # Comprehensions/os_walk_comprehension.py
    import os
    restFiles = [os.path.join(d[0], f) for d in os.walk(".")
                 for f in d[2] if f.endswith(".rst")]
    for r in restFiles:

A More Complex Example

..  note:: This will get a full description of all parts.


    # CodeManager.py
    TODO: Break check into two pieces?
    TODO: update() is still only in test mode; doesn't actually work yet.

    Extracts, displays, checks and updates code examples in restructured text (.rst)

    You can just put in the codeMarker and the (indented) first line (containing the
    file path) into your restructured text file, then run the update program to
    automatically insert the rest of the file.
    import os, re, sys, shutil, inspect, difflib

    restFiles = [os.path.join(d[0], f) for d in os.walk(".") if not "_test" in d[0]
                 for f in d[2] if f.endswith(".rst")]

    class Languages:
        "Strategy design pattern"

        class Python:
            codeMarker = "::\n\n"
            commentTag = "#"
            listings = re.compile("::\n\n( {4}#.*(?:\n+ {4}.*)*)")

        class Java:
            codeMarker = "..  code-block:: java\n\n"
            commentTag = "//"
            listings = \
                re.compile(".. *code-block:: *java\n\n( {4}//.*(?:\n+ {4}.*)*)")

    def shift(listing):
        "Shift the listing left by 4 spaces"
        return [x[4:] if x.startswith("    ") else x for x in listing.splitlines()]

    # TEST - makes duplicates of the rst files in a test directory to test update():
    dirs = set([os.path.join("_test", os.path.dirname(f)) for f in restFiles])
    if [os.makedirs(d) for d in dirs if not os.path.exists(d)]:
        [shutil.copy(f, os.path.join("_test", f)) for f in restFiles]
    testFiles = [os.path.join(d[0], f) for d in os.walk("_test")
                 for f in d[2] if f.endswith(".rst")]

    class Commands:
        Each static method can be called from the command line. Add a new static
        method here to add a new command to the program.

        def display(language):
            Print all the code listings in the .rst files.
            for f in restFiles:
                listings = language.listings.findall(open(f).read())
                if not listings: continue
                print('=' * 60 + "\n" + f + "\n" + '=' * 60)
                for n, l in enumerate(listings):
                    if n < len(listings) - 1:
                        print('-' * 60)

        def extract(language):
            Pull the code listings from the .rst files and write each listing into
            its own file. Will not overwrite if code files and .rst files disagree
            unless you say "extract -force".
            force = len(sys.argv) == 3 and sys.argv[2] == '-force'
            paths = set()
            for listing in [shift(listing) for f in restFiles
                        for listing in language.listings.findall(open(f).read())]:
                path = listing[0][len(language.commentTag):].strip()
                if path in paths:
                    print("ERROR: Duplicate file name: %s" % path)
                path = os.path.join("..", "code", path)
                dirname = os.path.dirname(path)
                if dirname and not os.path.exists(dirname):
                if os.path.exists(path) and not force:
                    for i in difflib.ndiff(open(path).read().splitlines(), listing):
                        if i.startswith("+ ") or i.startswith("- "):
                            print("ERROR: Existing file different from .rst")
                            print("Use 'extract -force' to force overwrite")
                file(path, 'w').write("\n".join(listing))

        def check(language):
            Ensure that external code files exist and check which external files
            have changed from what's in the .rst files. Generate files in the
            _deltas subdirectory showing what has changed.
            class Result: # Messenger
                def __init__(self, **kwargs):
                    self.__dict__ = kwargs
            result = Result(missing = [], deltas = [])
            listings = [Result(code = shift(code), file = f)
                        for f in restFiles for code in
            paths = [os.path.normpath(os.path.join("..", "code", path)) for path in
                         for listing in listings]]
            if os.path.exists("_deltas"):
            for path, listing in zip(paths, listings):
                if not os.path.exists(path):
                    code = open(path).read().splitlines()
                    for i in difflib.ndiff(listing.code, code):
                        if i.startswith("+ ") or i.startswith("- "):
                            d = difflib.HtmlDiff()
                            if not os.path.exists("_deltas"):
                            html = os.path.join("_deltas",
                                os.path.basename(path).split('.')[0] + ".html")
                            open(html, 'w').write(
                                "<html><h1>Left: %s<br>Right: %s</h1>" %
                                (listing.file, path) +
                                d.make_file(listing.code, code))
                            result.deltas.append(Result(file = listing.file,
                                path = path, html = html, code = code))
            if result.missing:
                print("Missing %s files:\n%s" %
                      (language.__name__, "\n".join(result.missing)))
            for delta in result.deltas:
                print("%s changed in %s; see %s" %
                      (delta.file, delta.path, delta.html))
            return result

        def update(language): # Test until it is trustworthy
            Refresh external code files into .rst files.
            check_result = Commands.check(language)
            if check_result.missing:
                print(language.__name__, "update aborted")
            changed = False
            def _update(matchobj):
                listing = shift(matchobj.group(1))
                path = listing[0].strip()[len(language.commentTag):].strip()
                filename = os.path.basename(path).split('.')[0]
                path = os.path.join("..", "code", path)
                code = open(path).read().splitlines()
                return language.codeMarker + \
                    "\n".join([("    " + line).rstrip() for line in listing])
            for f in testFiles:
                updated = language.listings.sub(_update, open(f).read())
                open(f, 'w').write(updated)

    if __name__ == "__main__":
        commands = dict(inspect.getmembers(Commands, inspect.isfunction))
        if len(sys.argv) < 2 or sys.argv[1] not in commands:
            print("Command line options:\n")
            for name in commands:
                print(name + ": " + commands[name].__doc__)
            for language in inspect.getmembers(Languages, inspect.isclass):