support FIPS builds without SHA-1 (#378)

CHANGES.rst

@@ -13,6 +13,9 @@ Unreleased
     By default it is ``Serializer[str]`` and ``dumps`` returns a ``str``. If a
     different ``serializer`` argument is given, it will try to infer the return
     type of its ``dumps`` method. :issue:`347`
+-   The default ``hashlib.sha1`` may not be available in FIPS builds. Don't
+    access it at import time so the developer has time to change the default.
+    :issue:`375`
 
 
 Version 2.1.2

src/itsdangerous/signer.py

@@ -37,13 +37,21 @@ def get_signature(self, key: bytes, value: bytes) -> bytes:
         return b""
 
 
+def _lazy_sha1(string: bytes = b"") -> t.Any:
+    """Don't access ``hashlib.sha1`` until runtime. FIPS builds may not include
+    SHA-1, in which case the import and use as a default would fail before the
+    developer can configure something else.
+    """
+    return hashlib.sha1(string)
+
+
 class HMACAlgorithm(SigningAlgorithm):
     """Provides signature generation using HMACs."""
 
     #: The digest method to use with the MAC algorithm. This defaults to
     #: SHA1, but can be changed to any other function in the hashlib
     #: module.
-    default_digest_method: t.Any = staticmethod(hashlib.sha1)
+    default_digest_method: t.Any = staticmethod(_lazy_sha1)
 
     def __init__(self, digest_method: t.Any = None):
         if digest_method is None:
@@ -109,7 +117,7 @@ class Signer:
     #: doesn't apply when used intermediately in HMAC.
     #:
     #: .. versionadded:: 0.14
-    default_digest_method: t.Any = staticmethod(hashlib.sha1)
+    default_digest_method: t.Any = staticmethod(_lazy_sha1)
 
     #: The default scheme to use to derive the signing key from the
     #: secret key and salt. The default is ``django-concat``. Possible

tests/test_itsdangerous/test_serializer.py

@@ -14,6 +14,7 @@
 from itsdangerous.exc import BadPayload
 from itsdangerous.exc import BadSignature
 from itsdangerous.serializer import Serializer
+from itsdangerous.signer import _lazy_sha1
 from itsdangerous.signer import Signer
 
 
@@ -177,7 +178,7 @@ class Signer256(serializer.signer):  # type: ignore
         )
 
         unsigners = serializer.iter_unsigners()
-        assert next(unsigners).digest_method == hashlib.sha1
+        assert next(unsigners).digest_method == _lazy_sha1
 
         for signer in unsigners:
             assert signer.digest_method == hashlib.sha256