Getopt - A Case Study in Reading Source

Sat 19 July 2014

Filed under Blog

Tags practice reading command-line-parser

To get better at reading source code, you just have to do it more often.

Here is an example annotation that I did while reading the source for getopt, a deprecated Python Standard Library module. Despite the simplicity, I picked up a few golden stylistic tips and abstraction techniques that I still use in my programming today.

All of my annotations start with ##. The original author's comments is written with #.

"""Parser for command line options.

This module helps scripts to parse the command line arguments in
sys.argv.  It supports the same conventions as the Unix getopt()
function (including the special meanings of arguments of the form `-'
and `--').  Long options similar to those supported by GNU software
may be used as well via an optional third argument.  This module
provides two functions and an exception:

getopt() -- Parse command line options
gnu_getopt() -- Like getopt(), but allow option and non-option arguments
to be intermixed.
GetoptError -- exception (class) raised with 'opt' attribute, which is the
option involved with the exception.

# Long option support added by Lars Wirzenius <>.
# Gerrit Holl <> moved the string-based exceptions
# to class-based exceptions.
# Peter ├ůstrand <> added gnu_getopt().
# TODO for gnu_getopt():
# - GNU getopt_long_only mechanism
# - allow the caller to specify ordering
# - RETURN_IN_ORDER option
# - GNU extension with '-' as first character of option string
# - optional arguments, specified by double colons
# - a option string with a W followed by semicolon should
#   treat "-W foo" as "--foo"

__all__ = ["GetoptError","error","getopt","gnu_getopt"]

import os

class GetoptError(Exception):
    opt = ''
    msg = ''
    def __init__(self, msg, opt=''):
        self.msg = msg
        self.opt = opt
        Exception.__init__(self, msg, opt)

    def __str__(self):
        return self.msg

error = GetoptError # backward compatibility

def getopt(args, shortopts, longopts = []):
    """getopt(args, options[, long_options]) -> opts, args

    Parses command line options and parameter list.  args is the
    argument list to be parsed, without the leading reference to the
    running program.  Typically, this means "sys.argv[1:]".  shortopts
    is the string of option letters that the script wants to
    recognize, with options that require an argument followed by a
    colon (i.e., the same format that Unix getopt() uses).  If
    specified, longopts is a list of strings with the names of the
    long options which should be supported.  The leading '--'
    characters should not be included in the option name.  Options
    which require an argument should be followed by an equal sign

    The return value consists of two elements: the first is a list of
    (option, value) pairs; the second is the list of program arguments
    left after the option list was stripped (this is a trailing slice
    of the first argument).  Each option-and-value pair returned has
    the option as its first element, prefixed with a hyphen (e.g.,
    '-x'), and the option argument as its second element, or an empty
    string if the option has no argument.  The options occur in the
    list in the same order in which they were found, thus allowing
    multiple occurrences.  Long and short options may be mixed.


    ## opts holds all options parsed so far
    ## args holds all unparsed data

    opts = []
    if type(longopts) == type(""):
        longopts = [longopts]
        longopts = list(longopts)
    while args and args[0].startswith('-') and args[0] != '-':

    ## We are in something like:
    ## -a -b x -c --ef - <positional> <positional>
    ## This loop instantly stops at the - or any none 
    ## flagged (starts with a -) argument.
    ## Loop Invariant: All options seen have been 
    ## successfully parsed and stored in opts.
    ## Exit Condition: Either no arguments left (arg is empty) 
    ## or next argument to be 
    ## parsed is either a positional, '-', or '--' argument 
    ## Main steps: Consume one optional parameter

        if args[0] == '--':

    ## Stop at '--' but DON'T include it in the returned arguments
    ## i.e '-a -b -- c d' returns options=(a,b) and args=(c,d) 
    ## if it was '-a -b - c d' you would get options=(a,b) args=(-, c, d)

        args = args[1:]
        if args[0].startswith('--'):

    ## handle '--long_option' case
    ## Strip off the '--' from the argument before passing it to do_longs

            opts, args = do_longs(opts, args[0][2:], longopts, args[1:])

    ## Handle '-o' case
    ## Pass in opts so far, first optional arg w/o the leading '-',
    ##  handled shortopts, all other args

            opts, args = do_shorts(opts, args[0][1:], shortopts, args[1:])

    return opts, args

def do_longs(opts, opt, longopts, args):

    ## If = is in the argument, this opt should take an argument

        i = opt.index('=')
    except ValueError:

    ## None = means no argument

        optarg = None

    ## Split the argument into parts. opt is stuff before the '=' and 
    ## optarg is stuff after the =.
    ## E.g opt="outfile=result.txt" --> opt = "outfile" optarg = "result.txt"

        opt, optarg = opt[:i], opt[i+1:]

    # Check that this specific opt should take an argument
    has_arg, opt = long_has_args(opt, longopts)
    if has_arg:
        if optarg is None:
            if not args:

    ## No more arguments after this long option.
    ##  We didn't get the argument we expected

                raise GetoptError('option --%s requires argument' % opt, opt)

    ## If there is an argument, put it in optarg and move arg forward

            optarg, args = args[0], args[1:]
    elif optarg:

    ## Got argument when we didn't expect one

        raise GetoptError('option --%s must not have an argument' % opt, opt)

    ## Append in optarg with appropriate argument

    opts.append(('--' + opt, optarg or ''))
    return opts, args

# Return:
#   has_arg?
#   full option name
def long_has_args(opt, longopts):

    ## Returns has_arg (True | False)
    ## Returns the full option name given a shorter prefix

    possibilities = [o for o in longopts if o.startswith(opt)]
    if not possibilities:
        raise GetoptError('option --%s not recognized' % opt, opt)
    # Is there an exact match?
    if opt in possibilities:
        return False, opt
    elif opt + '=' in possibilities:
        return True, opt
    # No exact match, so better be unique.
    if len(possibilities) > 1:
        # XXX since possibilities contains all valid continuations, might be
        # nice to work them into the error msg
        raise GetoptError('option --%s not a unique prefix' % opt, opt)

    ## Assert is clearly not necessary because we checked 
    ## possibilities == 0 and possibilities > 1
    ## Serves as a clever comment

    assert len(possibilities) == 1
    unique_match = possibilities[0]
    has_arg = unique_match.endswith('=')
    if has_arg:
        unique_match = unique_match[:-1]
    return has_arg, unique_match

def do_shorts(opts, optstring, shortopts, args):

    ## optstring is a short argument or a list of short arguments like
    ##  -abcd with the '-' prefix removed
    ## Process the whole piece

    while optstring != '':

        ## Remove the first letter from the optstring

        opt, optstring = optstring[0], optstring[1:]

    ## Check if opt is even an argument

        if short_has_arg(opt, shortopts):

    ## If the optstring is followed by an argument

            if optstring == '':

    ## We are parsing a flag with an argument
    ## This is the case where there is a space between the flag 
    ## and the argument - like -o out.txt

                if not args:
                    raise GetoptError('option -%s requires argument' % opt,

    ## Grab the subsequent argument right after this flag to be the arg to the flag

                optstring, args = args[0], args[1:]

    ## If there is stuff after the flag, all of it is the argument

            optarg, optstring = optstring, ''

        ## If the opstring is standalone i.e 'v' from '-v'

            optarg = ''
        opts.append(('-' + opt, optarg))
    return opts, args

def short_has_arg(opt, shortopts):
    ## Check that opt exists and requires an arg
    for i in range(len(shortopts)):
        if opt == shortopts[i] != ':':
            return shortopts.startswith(':', i+1)
    raise GetoptError('option -%s not recognized' % opt, opt)

if __name__ == '__main__':
    import sys
    print getopt(sys.argv[1:], "a:b", ["alpha=", "beta"])

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