Django cannot load when Python is compiled with --without-doc-strings enabled

[Jon Janzen]

I'm not sure that this is even a supported configuration for Django, but CPython's ./configure script offers an option called ​--without-doc-strings which improves memory utilization slightly.

Quoting the relevant portion from CPython docs:

--without-doc-strings
Disable static documentation strings to reduce the memory footprint (enabled by default). Documentation strings defined in Python are not affected.

I have an use-case where I need to deploy a Django service on a device with low available memory. I'd like to disable built-in doc strings as part of an overall strategy to reduce memory but compiling CPython with that option and running the django service crashes:

  File ".../asgi.py", line 3, in <module>
    from django.core.asgi import get_asgi_application
  File ".../lib/python3.11/site-packages/django/core/asgi.py", line 2, in <module>
    from django.core.handlers.asgi import ASGIHandler
  File ".../lib/python3.11/site-packages/django/core/handlers/asgi.py", line 11, in <module>
    from django.core.handlers import base
  File ".../lib/python3.11/site-packages/django/core/handlers/base.py", line 11, in <module>
    from django.urls import get_resolver, set_urlconf
  File ".../lib/python3.11/site-packages/django/urls/__init__.py", line 1, in <module>
    from .base import (
  File ".../lib/python3.11/site-packages/django/urls/base.py", line 8, in <module>
    from .exceptions import NoReverseMatch, Resolver404
  File ".../lib/python3.11/site-packages/django/urls/exceptions.py", line 1, in <module>
    from django.http import Http404
  File ".../lib/python3.11/site-packages/django/http/__init__.py", line 2, in <module>
    from django.http.request import (
  File ".../lib/python3.11/site-packages/django/http/request.py", line 8, in <module>
    from django.core import signing
  File ".../lib/python3.11/site-packages/django/core/signing.py", line 43, in <module>
    from django.utils.crypto import constant_time_compare, salted_hmac
  File ".../lib/python3.11/site-packages/django/utils/crypto.py", line 85, in <module>
    if func_supports_parameter(hashlib.md5, 'usedforsecurity'):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.11/site-packages/django/utils/inspect.py", line 74, in func_supports_parameter
    return any(param.name == name for param in _get_callable_parameters(func))
                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.11/site-packages/django/utils/inspect.py", line 16, in _get_callable_parameters
    return _get_func_parameters(func, remove_first=is_method)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.11/site-packages/django/utils/inspect.py", line 7, in _get_func_parameters
    parameters = tuple(inspect.signature(func).parameters.values())
                       ^^^^^^^^^^^^^^^^^^^^^^^
  File ".../cpython/Lib/inspect.py", line 3270, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../cpython/Lib/inspect.py", line 3018, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../cpython/Lib/inspect.py", line 2510, in _signature_from_callable
    return _signature_from_builtin(sigcls, obj,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../cpython/Lib/inspect.py", line 2317, in _signature_from_builtin
    raise ValueError("no signature found for builtin {!r}".format(func))
ValueError: no signature found for builtin <built-in function openssl_md5>

(irrelevant information lightly censored)

An easy way to repro is to follow this script:

git clone https://github.com/python/cpython.git
cd cpython
git checkout 3.11 # or whatever version you have that already has django installed
./configure --without-doc-strings
make -j
./python -c "import django.utils.crypto"

Looking through this stack trace the problem looks to be ​some code that executes at module import to determine whether or not md5 supports the usedforsecurity parameter.

If this ticket is accepted I'm happy to put up a PR to fix this, I've included my proposed solution on this ticket to save a roundtrip in discussion:

diff --git a/django/utils/inspect.py b/django/utils/inspect.py
index 28418f7312..d8622a22df 100644
--- a/django/utils/inspect.py
+++ b/django/utils/inspect.py
@@ -70,4 +70,12 @@ def method_has_no_args(meth):
 
 
 def func_supports_parameter(func, name):
-    return any(param.name == name for param in _get_callable_parameters(func))
+    try:
+        callable_parameters = _get_callable_parameters(func)
+    except ValueError:
+        # When Python is compiled with the --without-doc-strings
+        # argument to ./configure the signatures for built-in
+        # functions are not available. In such a case, be
+        # conservative and assume no:
+        return False
+    return any(param.name == name for param in callable_parameters)