Script to remove Python comments/docstrings

Question

Is there a Python script or tool available which can remove comments and docstrings from Python source?

It should take care of cases like:

"""
aas
"""
def f():
    m = {
        u'x':
            u'y'
        } # faake docstring ;)
    if 1:
        'string' >> m
    if 2:
        'string' , m
    if 3:
        'string' > m

---

So at last I have come up with a simple script, which uses the tokenize module and removes comment tokens. It seems to work pretty well, except that I am not able to remove docstrings in all cases. See if you can improve it to remove docstrings.

import cStringIO
import tokenize

def remove_comments(src):
    """
    This reads tokens using tokenize.generate_tokens and recombines them
    using tokenize.untokenize, and skipping comment/docstring tokens in between
    """
    f = cStringIO.StringIO(src)
    class SkipException(Exception): pass
    processed_tokens = []
    last_token = None
    # go thru all the tokens and try to skip comments and docstrings
    for tok in tokenize.generate_tokens(f.readline):
        t_type, t_string, t_srow_scol, t_erow_ecol, t_line = tok

        try:
            if t_type == tokenize.COMMENT:
                raise SkipException()

            elif t_type == tokenize.STRING:

                if last_token is None or last_token[0] in [tokenize.INDENT]:
                    # FIXEME: this may remove valid strings too?
                    #raise SkipException()
                    pass

        except SkipException:
            pass
        else:
            processed_tokens.append(tok)

        last_token = tok

    return tokenize.untokenize(processed_tokens)

Also I would like to test it on a very large collection of scripts with good unit test coverage. Can you suggest such a open source project?

Answer 1

I'm the author of the "mygod, he has written a python interpreter using regex..." (i.e. pyminifier) mentioned at that link below =).
I just wanted to chime in and say that I've improved the code quite a bit using the tokenizer module (which I discovered thanks to this question =) ).

You'll be happy to note that the code no longer relies so much on regular expressions and uses tokenizer to great effect. Anyway, here's the remove_comments_and_docstrings() function from pyminifier
(Note: It works properly with the edge cases that previously-posted code breaks on):

import cStringIO, tokenize
def remove_comments_and_docstrings(source):
    """
    Returns 'source' minus comments and docstrings.
    """
    io_obj = cStringIO.StringIO(source)
    out = ""
    prev_toktype = tokenize.INDENT
    last_lineno = -1
    last_col = 0
    for tok in tokenize.generate_tokens(io_obj.readline):
        token_type = tok[0]
        token_string = tok[1]
        start_line, start_col = tok[2]
        end_line, end_col = tok[3]
        ltext = tok[4]
        # The following two conditionals preserve indentation.
        # This is necessary because we're not using tokenize.untokenize()
        # (because it spits out code with copious amounts of oddly-placed
        # whitespace).
        if start_line > last_lineno:
            last_col = 0
        if start_col > last_col:
            out += (" " * (start_col - last_col))
        # Remove comments:
        if token_type == tokenize.COMMENT:
            pass
        # This series of conditionals removes docstrings:
        elif token_type == tokenize.STRING:
            if prev_toktype != tokenize.INDENT:
        # This is likely a docstring; double-check we're not inside an operator:
                if prev_toktype != tokenize.NEWLINE:
                    # Note regarding NEWLINE vs NL: The tokenize module
                    # differentiates between newlines that start a new statement
                    # and newlines inside of operators such as parens, brackes,
                    # and curly braces.  Newlines inside of operators are
                    # NEWLINE and newlines that start new code are NL.
                    # Catch whole-module docstrings:
                    if start_col > 0:
                        # Unlabelled indentation means we're inside an operator
                        out += token_string
                    # Note regarding the INDENT token: The tokenize module does
                    # not label indentation inside of an operator (parens,
                    # brackets, and curly braces) as actual indentation.
                    # For example:
                    # def foo():
                    #     "The spaces before this docstring are tokenize.INDENT"
                    #     test = [
                    #         "The spaces before this string do not get a token"
                    #     ]
        else:
            out += token_string
        prev_toktype = token_type
        last_col = end_col
        last_lineno = end_line
    return out