Make db_transaction* remove db/cur from the signature.

Rremoving them allows testing the function's signature
matches the existing signature of a specification and type checking.

Moreover, they should not be used by users of the class, so there is no
reason to have them appear in the documentation (generated from
the signature).

swh/core/db/common.py

@@ -7,6 +7,19 @@ import inspect
 import functools
 
 
+def remove_kwargs(names):
+    def decorator(f):
+        sig = inspect.signature(f)
+        params = sig.parameters
+        params = [param for param in params.values()
+                  if param.name not in names]
+        sig = sig.replace(parameters=params)
+        f.__signature__ = sig
+        return f
+
+    return decorator
+
+
 def apply_options(cursor, options):
     """Applies the given postgresql client options to the given cursor.
 
@@ -33,6 +46,7 @@ def db_transaction(**client_options):
             raise ValueError(
                     'Use db_transaction_generator for generator functions.')
 
+        @remove_kwargs(['cur', 'db'])
         @functools.wraps(meth)
         def _meth(self, *args, **kwargs):
             if 'cur' in kwargs and kwargs['cur']:
@@ -67,6 +81,7 @@ def db_transaction_generator(**client_options):
             raise ValueError(
                     'Use db_transaction for non-generator functions.')
 
+        @remove_kwargs(['cur', 'db'])
         @functools.wraps(meth)
         def _meth(self, *args, **kwargs):
             if 'cur' in kwargs and kwargs['cur']:

swh/core/db/tests/test_db.py

@@ -3,6 +3,7 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+import inspect
 import os.path
 import tempfile
 import unittest
@@ -151,6 +152,21 @@ def test_db_transaction__with_generator():
                 yield None
 
 
+def test_db_transaction_signature():
+    """Checks db_transaction removes the 'cur' and 'db' arguments."""
+    def f(self, foo, *, bar=None):
+        pass
+    expected_sig = inspect.signature(f)
+
+    @db_transaction()
+    def g(self, foo, *, bar=None, db=None, cur=None):
+        pass
+
+    actual_sig = inspect.signature(g)
+
+    assert actual_sig == expected_sig
+
+
 def test_db_transaction_generator(mocker):
     expected_cur = object()
 
@@ -189,3 +205,18 @@ def test_db_transaction_generator__with_nongenerator():
             @db_transaction_generator()
             def endpoint(self, cur=None, db=None):
                 pass
+
+
+def test_db_transaction_generator_signature():
+    """Checks db_transaction removes the 'cur' and 'db' arguments."""
+    def f(self, foo, *, bar=None):
+        pass
+    expected_sig = inspect.signature(f)
+
+    @db_transaction_generator()
+    def g(self, foo, *, bar=None, db=None, cur=None):
+        yield None
+
+    actual_sig = inspect.signature(g)
+
+    assert actual_sig == expected_sig