[New-bugs-announce] [issue34226] cgi.parse_multipart() requires undocumented CONTENT-LENGTH in Python 3.7

[Chih-Hsuan Yen]

New submission from Chih-Hsuan Yen <yan12125 at gmail.com>:

In Python 3.7, cgi.parse_multipart() requires "CONTENT-LENGTH" in the second parameter `pdict`. This is not necesary in Python 3.6.

For example, the following code runs with 3.6:

$ python3.6
Python 3.6.6 (default, Jun 27 2018, 13:11:40) 
[GCC 8.1.1 20180531] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cgi, io
>>> data = b'--heyDavid\r\nContent-Disposition: form-data; name="cfield"\r\n\r\njust a string\r\n\r\n--heyDavid--\r\n'
>>> cgi.parse_multipart(io.BytesIO(data), {"boundary": b"heyDavid"})
{'cfield': [b'just a string\r\n']}

While not on 3.7:

$ python3.7
Python 3.7.0 (default, Jul 15 2018, 10:44:58) 
[GCC 8.1.1 20180531] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cgi, io
>>> data = b'--heyDavid\r\nContent-Disposition: form-data; name="cfield"\r\n\r\njust a string\r\n\r\n--heyDavid--\r\n'
>>> cgi.parse_multipart(io.BytesIO(data), {"boundary": b"heyDavid"})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.7/cgi.py", line 220, in parse_multipart
    headers['Content-Length'] = pdict['CONTENT-LENGTH']
KeyError: 'CONTENT-LENGTH'

I looked into the source code of CPython, and found CONTENT-LENGTH in nowhere but cgi.py and test_cgi.py. I guess it has the same meaning as the Content-Length header in HTTP or CONTENT_LENGTH environment variable in CGI? It would be great to document such a backward-incompatible behavior.

Environment: Arch Linux with Python 3.6.6 from [extra] repo and 3.7.0 from [staging] repo.

CC the author and the reviewer of issue29979, where relevant codes are introduced.

----------
assignee: docs at python
components: Documentation, Library (Lib)
messages: 322362
nosy: docs at python, orsenthil, quentel, yan12125
priority: normal
severity: normal
status: open
title: cgi.parse_multipart() requires undocumented CONTENT-LENGTH in Python 3.7
type: behavior
versions: Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34226>
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