Python requests module slow due to high number of internal function calls

Question

On one of my machines the Python module requests is extremely slow, seemingly due to a very high number of internal function calls. What could lead to such an issue?

>>> import sys, cProfile, requests, urllib3
>>> sys.version
'3.10.6 (main, Mar 10 2023, 10:55:28) [GCC 11.3.0]'
>>> requests.__version__
'2.28.1' # same issue with latest 2.29.0 release

>>> cProfile.run('requests.get("http://127.0.0.1:8080")', sort='ncalls')
        11717750 function calls (11717748 primitive calls) in 10.091 seconds

    Ordered by: call count

    ncalls  tottime  percall  cumtime  percall filename:lineno(function)
   5220221    0.770   0.000     0.775    0.000 {method 'read' of '_io.TextIOWrapper' objects}
   5220221    0.801   0.000     0.801    0.000 shlex.py:68(punctuation_chars)
    468483    0.364   0.000     9.564    0.000 shlex.py:101(get_token)
    401557    7.612   0.000     9.188    0.000 shlex.py:133(read_token)
    200797    0.059   0.000     0.059    0.000 {method 'startswith' of 'str' objects}
     66931    0.012   0.000     0.012    0.000 {method 'popleft' of 'collections.deque' objects}
     66926    0.012   0.000     0.012    0.000 {method 'appendleft' of 'collections.deque' objects}
     66926    0.049   0.000     0.061    0.000 shlex.py:72(push_token)
      [..] # remaining calls with significantly lower numbers

>>> cProfile.run('http=urllib3.PoolManager(); http.request("GET", "http://127.0.0.1:8080")', sort='ncalls')
        784 function calls (783 primitive calls) in 0.005 seconds

On another machines, running the same version of Python and tested with same versions of requests, things look about right:

>>> cProfile.run('requests.get("http://127.0.0.1:8080")', sort='ncalls')
        2432 function calls (2396 primitive calls) in 0.005 seconds

• Above tests were done using python3 -m http.server 8080 --bind 127.0.0.1, but same results for other HTTP servers
• Most of the time spent is before a TCP connection is even created by looking at network traffic using tcpdump
• As with urllib3, completely unrelated tools such as curl works just fine
• While tests were made on IPv4 addresses, actively disabling IPv6 for requests using requests.packages.urllib3.util.connection.HAS_IPV6 = False did not have any effect either
• Tested with clean virtualenvs

Answer 1

Using the Python profile viewer tuna I found that requests will attempt to get authentication credentials from a netrc file ($HOME/.netrc or $HOME/_netrc) if no other authentication method is specified:

• https://requests.readthedocs.io/en/latest/user/authentication/#netrc-authentication
• https://github.com/psf/requests/blob/v2.28.1/requests/sessions.py#L478

Instead of just looking at number of calls (ncalls), sorting by cumulative time (sort=cumtime) would also have helped highlight how the issuing of netrc.py and subsequent shlex.py were the main culprits.

In my case I had a fairly large netrc file at thousands of entries. Upon trimming or removing the netrc file entirely the subsequent number of function calls shrunk accordingly.

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Ideally the performance of the netrc Python module wouldn't be such a bottleneck, as the file itself isn't all that complex, and other programs like curl has no issues handling it, but oh well.