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Sending requests with Content-Length but without Content-Disposition headers causes following error:
Traceback (most recent call last):
 File "./test", line 19, in <module>
 form = cgi.FieldStorage(fp=env['wsgi.input'], environ=env)
 File "/usr/lib/python3.5/cgi.py", line 561, in __init__
 self.read_single()
 File "/usr/lib/python3.5/cgi.py", line 740, in read_single
 self.read_binary()
 File "/usr/lib/python3.5/cgi.py", line 762, in read_binary
 self.file.write(data)
TypeError: write() argument must be str, not bytes
I've attached a test file that reproduces the issue.
The issue is because read_single decides whether to read the content as binary or text depending on content-length - if it's > 0, it uses read_binary which assumes binary input, and rewrites this input to self.file, assuming self.file is opened in binary mode.
At the same, self.file is opened in text mode, because self._binary_file is set to False, which in turn is because there's no Content-Disposition header.
At very least, the decision whether to use binary or text should be consistent in both places (self.length >= 0 vs self._binary_file).
Related: https://bugs.python.org/issue27308
Note that unlike https://bugs.python.org/issue24764 this issue does NOT concern multipart requests.