[Patch #101120] add .get() to cgi.FieldStorage and cgi.FormContentDict

[Ka-Ping Yee]

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Hi, guys.

This patch includes a "fix" to make cgi.parse_qsl honour its
"keep_blank_values" argument.  This fix might require a bit more
discussion since it could break compatibility, so i thought i
would air it for your consideration.


BACKGROUND
----------

cgi.FieldStorage(), cgi.parse(), cgi.parse_qs(), and cgi.parse_qsl()
all accept an optional "keep_blank_values" argument which is supposed
to indicate whether form fields with empty strings as values should
be included in the result.  In reality, only cgi.parse_qs() honours
this flag (cgi.parse() is okay because it uses cgi.parse_qs()).
cgi.parse_qsl() totally ignores it, and always includes all values
including empty ones.  cgi.FieldStorage(), which uses cgi.parse_qsl(),
similarly does not honour the "keep_blank_values" flag.

The patch contains a fix to make cgi.parse_qsl() honour this flag.


FOR
---

It's clear that this was the intent of the "keep_blank_values"
argument.  The doc string for parse_qsl says:

    """Parse a query given as a string argument.

        Arguments:

        qs: URL-encoded query string to be parsed

        keep_blank_values: flag indicating whether blank values in
            URL encoded queries should be treated as blank strings.­­
            A true value indicates that blanks should be retained as­
            blank strings.  The default false value indicates that
            blank values are to be ignored and treated as if they were
            not included.

        strict_parsing: flag indicating what to do with parsing errors.
            If false (the default), errors are silently ignored.
            If true, errors raise a ValueError exception.

       Returns a list, as God intended.
    """

Its current disregard for "keep_blank_values" clearly contradicts
this doc string.

The doc string for FieldStorage.__init__ says:

        """Constructor.  Read multipart/* until last part.

        Arguments, all optional:

        fp              : file pointer; default: sys.stdin
            (not used when the request method is GET)

        headers         : header dictionary-like object; default:
            taken from environ as per CGI spec

        outerboundary   : terminating multipart boundary
            (for internal use only)

        environ         : environment dictionary; default: os.environ

        keep_blank_values: flag indicating whether blank values in
            URL encoded forms should be treated as blank strings.­­
            A true value indicates that blanks should be retained as­
            blank strings.  The default false value indicates that
            blank values are to be ignored and treated as if they were
            not included.

        strict_parsing: flag indicating what to do with parsing errors.
            If false (the default), errors are silently ignored.
            If true, errors raise a ValueError exception.

        """

...and the current behaviour of FieldStorage.__init__ contradicts
what it says here about "keep_blank_values".


AGAINST
-------

Existing code which uses FieldStorage and relies on its current
behaviour could break.  If a form field is left blank by the user
and the form is submitted, that key will not appear in the
FieldStorage -- and an attempt to index it with form[key] will 
produce a KeyError where previously it yielded a MiniFieldStorage
containing an empty string.

The existing TeX documentation for the cgi module does not mention
empty values or the keep_blank_values argument at all, so to those
who have only read the library reference manual, this could be a
surprise.


OPTIONS
-------

The reason this never showed up before is that there have never
been *any* tests for FieldStorage in test_cgi.py!  Naughty, naughty.

One easy way to maintain compatibility is to change FieldStorage so
that the default value for keep_blank_values is 1 (previously this
was 0 but the FieldStorage *acted* as though it were 1).


-- ?!ng