Signature changes

Various changes to the signature of a callable will produce errors.

Error codes

Code	Name
B300	removed-argument
B310	added-argument
B320	moved-argument
B330	unpositional-argument
B340	removed-var-args
B350	removed-var-keyword-args
B800	uncallable
N400	added-optional-argument
N410	added-argument-default
N440	added-var-args
N450	added-var-keyword-args

Examples

B300 removed-argument

-def fn(x, y, z)
+def fn(x, y)

# breaks: fn('x', 'y', 'z')

An argument was removed, breaking any clients who were formerly providing the argument.

B310 added-argument

-def fn(x, y)
+def fn(x, y, z)

# breaks: fn('x', 'y')

A mandatory argument was added, breaking all clients formerly using the function.

B320 moved-argument

-def scramble_eggs(with_butter=False, with_milk=True)
+def scramble_eggs(with_milk=True, with_butter=False)

# breaks: scramble_eggs(True, False)

The position of an argument was changed, breaking clients passing the relevant argument(s) positionally. Although, depending on the argument types, the call may be able to succeed, it’s likely the semantics of the call have changed in a backwards-incompatible manner.

B330 unpositional-argument

-def scramble_eggs(with_butter=False)
+def scramble_eggs(*eggs, with_butter=False)

# breaks: scramble_eggs(True)

An argument could previously be provided positionally, but can now only be passed as a keyword argument.

B340 removed-var-args

-def max(*values)
+def max(a, b)

# breaks: max(10, 3, 7)

A function previously accepted an unlimited number of positional arguments, and no longer does. This breaks any clients formerly passing more arguments than accepted by the new version of the function.

B350 removed-var-keyword-args

-def scramble_eggs(**kwargs)
+def scramble_eggs(with_butter=False, with_milk=False)

# breaks: scramble_eggs(with_salt=True)

A function previously accepted an unlimited number of keyword arguments, and no longer does. This breaks any clients formerly passing any keyword arguments other than those supported by the new version of the function.

B800 uncallable

 class EggScrambler:
-  def __call__(self): ...
+  def scramble_eggs(self): ...
 scrambler = EggScrambler()

# breaks: scrambler()

An object changed from callable to non-callable. This breaks any clients formerly using the object as a callable.

N400 added-optional-argument

-def scramble_eggs(with_butter=False)
+def scramble_eggs(with_butter=False, with_milk=True)

# compatible: scramble_eggs(True)
# new call:   scramble_eggs(True, True)

A named argument was added with a default value. Since a default is supplied, this is a backwards-compatible change.

N410 added-argument-default

-def scramble_eggs(with_butter)
+def scramble_eggs(with_butter=False)

# compatible: scramble_eggs(True)
# new call:   scramble_eggs()

An argument has a default value introduced. This is a backwards-compatible change which allows a function to be called with fewer arguments.

N440 added-var-args

-def max(a, b)
+def max(a, b, *rest)

# compatible: max(10, 3)
# new call:   max(10, 3, 7)

A function now accepts any number of positional arguments.

N450 added-var-keyword-args

-def scramble_eggs(with_butter=True)
+def scramble_eggs(with_butter=True, **kwargs)

# compatible: scramble_eggs(True)
# new call:   scramble_eggs(True, with_salt=True)

A function now accepts any number of named arguments.