pytype

A static type analyzer for Python code

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Error classes

pytype has the following classes of errors, which can be disabled with a pytype: disable=error-class directive. For example, to suppress an error for a missing attribute foo:

x = a.foo  # pytype: disable=attribute-error

or, to suppress all attribute errors for a block of code:

# pytype: disable=attribute-error
x = a.foo
y = a.bar
# pytype: enable=attribute-error

See Silencing Errors for a more detailed example.

annotation-type-mismatch

A variable had a type annotation and an assignment with incompatible types.

Example:

x: int = 'hello world'

This error is also raised for wrongly assigning a value in a TypedDict:

from typing import TypedDict

class A(TypedDict):
  x: int
  y: str

a = A()
a['x'] = '10'

assert-type

The error message displays the expected and actual type of the expression passed to assert_type() if the two do not match. Example:

x = 10
assert_type(x, str)

will raise the error

File "foo.py", line 2, in <module>:
Type[int] [assert-type]
Expected: str
  Actual: int

The expected type can be either a python type (like str or foo.A) or its string representation. The latter form is useful when you want to assert a type without importing it, e.g.

from typing import List

assert_type(x, List[int])

versus

assert_type(x, 'List[int]')

assert_type can also be used without an expected argument to assert that a type is not Any; in that case the error message is

File "foo.py", line 10, in f: Asserted type was Any [assert-type]

attribute-error

The attribute being accessed may not exist. Often, the reason is that the attribute is declared in a method other than __new__ or __init__:

class A(object):
  def make_foo(self):
    self.foo = 42
  def consume_foo(self):
    return self.foo  # attribute-error

To make pytype aware of foo, declare its type with a variable annotation:

class A(object):
  foo: int

NOTE: This declaration does not define the attribute at runtime.

bad-concrete-type

A generic type was instantiated with incorrect concrete types. Example:

from typing import Generic, TypeVar

T = TypeVar('T', int, float)

class A(Generic[T]):
  pass

obj = A[str]()  # bad-concrete-type

bad-function-defaults

An attempt was made to set the __defaults__ attribute of a function with an object that is not a constant tuple. Example:

import collections
X = collections.namedtuple("X", "a")
X.__new__.__defaults__ = [None]  # bad-function-defaults

bad-return-type

At least one of the possible types for the return value does not match the declared return type. Example:

def f(x) -> int:
  if x:
    return 42
  else:
    return None  # bad-return-type

NOTE: For the corner case of an empty function whose body is a docstring, use the block form of disable to suppress the error:

# pytype: disable=bad-return-type
def f() -> int:
  """Override in subclasses and return int."""
# pytype: enable=bad-return-type

bad-slots

An attempt was made to set the __slots__ attribute of a class using an object that’s not a string.

class Foo(object):
  __slots__ = (1, 2, 3)

bad-unpacking

A tuple was unpacked into the wrong number of variables. Example:

a, b = (1, 2, 3)  # bad-unpacking

bad-yield-annotation

A generator function (a function with a yield) was not annotated with an appropriate return type.

def gen() -> int:  # bad-yield-annotation
  yield 1
def gen() -> Iterator[int]
  # Could also use Generator or Iterable.
  yield 1

base-class-error

The class definition uses an illegal value for a base class. Example:

class A(42):  # base-class-error
  pass

container-type-mismatch

A method call violated the type annotation of a container by modifying its contained type.

Example:

a: List[int] = [1, 2]
a.append("hello")  # <-- contained type is now Union[int, str]

duplicate-keyword-argument

A positional argument was supplied again as a keyword argument. Example:

def f(x):
  pass
f(True, x=False)  # duplicate-keyword-argument

If you believe you are seeing this error due to a bug on pytype’s end, see this section for where the type information we use is located.

final-error

An attempt was made to subclass, override or reassign a final object or variable. The exact meaning of “final” is context dependent; see PEP 591 for the full details. Example:

class A:
  FOO: Final[int] = 10

class B(A):
  FOO = 20  # final-error

ignored-abstractmethod

The abc.abstractmethod decorator was used in a non-abstract class. Example:

import abc
class A(object):  # ignored-abstractmethod
  @abc.abstractmethod
  def f(self):
    pass

Add the abc.ABCMeta metaclass to fix this issue:

import abc
class A(metaclass=abc.ABCMeta):
  @abc.abstractmethod
  def f(self):
    pass

ignored-metaclass

A Python 2 metaclass declaration was found. Example:

class A(object):
  __metaclass__ = Meta

The fix is to switch to a Python 3-style metaclass:

class A(metaclass=Meta):
  ...

ignored-type-comment

A type comment was found on a line on which type comments are not allowed. Example:

def f():
  return 42  # type: float  # ignored-type-comment

import-error

The module being imported was not found.

incomplete-match

A pattern match over an enum did not cover all possible cases.

from enum import Enum
class Color(Enum):
  RED = 0
  GREEN = 1
  BLUE = 2

def f(x: Color):
  match x:  # incomplete-match
    case Color.RED:
      return 10
    case Color.GREEN:
      return 20

invalid-annotation

Something is wrong with this annotation. A common issue is a TypeVar that appears only once in a function signature:

from typing import TypeVar
T = TypeVar("T")
def f(x: T):  # bad: the TypeVar appears only once in the signature
  pass

A TypeVar is meaningful only when it appears multiple times in the same class/function, since it’s used to indicate that two or more values have the same type.

Other examples:

from typing import List, Union

condition = ...  # type: bool
class _Foo: ...
def Foo():
  return _Foo()

def f(x: List[int, str]):  # bad: too many parameters for List
  pass
def f(x: Foo):  # bad: not a type
  pass
def f(x: Union):  # bad: no options in the union
  pass
def f(x: int if condition else str):  # bad: ambiguous type
  pass

You will also see this error if you use a forward reference in typing.cast or pass a bad type to attr.ib:

import attr
import typing

v = typing.cast("A", None)  # invalid-annotation
class A(object):
  pass

@attr.s
class Foo(object):
  v = attr.ib(type=zip)  # invalid-annotation

The solutions are to use a type comment and to fix the type:

import attr

v = None  # type: "A"
class A(object):
  pass

@attr.s
class Foo(object):
  v = attr.ib(type=list)

invalid-directive

The error name is misspelled in a pytype disable/enable directive. Example with a misspelled name-error:

x = TypeDefinedAtRuntime  # pytype: disable=nmae-error  # invalid-directive

invalid-function-definition

An invalid function was constructed, typically with a decorator such as @dataclass. Example:

from dataclasses import dataclass

@dataclass
class A:
  x: int = 10
  y: str

which creates

def __init__(x: int = 10, y: str):
  ...

with a non-default argument following a default one.

invalid-function-type-comment

Something was wrong with this function type comment. Examples:

def f(x):
  # type: (int)  # bad: missing return type
  pass
def f(x):
  # type: () -> None  # bad: too few arguments
  pass
def f(x):
  # type: int -> None  # bad: missing parentheses
  pass

invalid-namedtuple-arg

The typename or one of the field names in the namedtuple definition is invalid. Field names:

Also, there can be no duplicate field names. The typename has the same requirements, except that it can start with “_”.

invalid-super-call

A call to super without any arguments (Python 3) is being made from an invalid context. A super call without any arguments should be made from a method or a function defined within a class. Also, the caller should have at least one positional argument.

invalid-typevar

Something was wrong with this TypeVar definition. Examples:

from typing import TypeVar
T = TypeVar("S")  # bad: storing TypeVar "S" as "T"
T = TypeVar(42)  # bad: using a non-str value for the TypeVar name
T = TypeVar("T", str)  # bad: supplying a single constraint (did you mean `bound=str`?)
T = TypeVar("T", 0, 100)  # bad: 0 and 100 are not types

late-directive

A # pytype: disable without a matching following enable or a # type: ignore appeared on its own line after the first top-level definition. Such a directive takes effect for the rest of the file, regardless of indentation, which is probably not what you want:

def f() -> bool:
  # pytype: disable=bad-return-type  # late-directive
  return 42

Two equally acceptable fixes:

def f() -> bool:
  return 42  # pytype: disable=bad-return-type
# pytype: disable=bad-return-type
def f() -> bool:
  return 42
# pytype: enable=bad-return-type

match-error

An invalid pattern matching construct was used (e.g. too many positional parameters)

missing-parameter

The function was called with a parameter missing. Example:

def add(x, y):
  return x + y
add(42)  # missing-parameter

If you believe you are seeing this error due to a bug on pytype’s end, see this section for where the type information we use is located.

module-attr

The module attribute being accessed may not exist. Example:

import sys
sys.nonexistent_attribute  # module-attr

mro-error

A valid method resolution order cannot be created for the class being defined. Often, the culprit is cyclic inheritance:

class A(object):
  pass
class B(object, A):  # mro-error
  pass

name-error

This name does not exist in the current namespace. Note that types like List, Dict, etc., need to be imported from the typing module:

MyListType = List[str]  # name-error
from typing import List

MyListType = List[str]

Note that a name from an outer namespace cannot be referenced if you redefine it in the current namespace, unless you use the global or nonlocal keyword:

def f():
  x = 0
  def g():
    x += 1  # name-error
def f():
  x = 0
  def g():
    nonlocal x
    x += 1

not-callable

The object being called or instantiated is not callable. Example:

x = 42
y = x()  # not-callable

not-indexable

The object being indexed is not indexable. Example:

tuple[3]  # not-indexable

not-instantiable

The class cannot be instantiated because it has abstract methods. Example:

import abc
class A(metaclass=abc.ABCMeta):
  @abc.abstractmethod
  def f(self):
    pass
A()  # not-instantiable

not-supported-yet

This feature is not yet supported by pytype.

The fix for the error “Calling TypeGuard function ‘foo’ with an arbitrary expression not supported yet” is to refactor the code passing an arbitrary expression to foo to pass in a local variable instead. For example:

# Before:
if foo(eggs[0]):
  do_something()

# After:
egg = eggs[0]
if foo(egg):
  do_something()

not-writable

The object an attribute was set on doesn’t have that attribute, or that attribute isn’t writable:

class Foo(object):
  __slots__ = ("x", "y")

Foo().z = 42  # not-writable

paramspec-error

A parameter specification variable was used incorrectly. Examples include only having a single ParamSpec in a function signature, and not exactly following the limited syntax for P.args and P.kwargs.

pyi-error

The pyi file contains a syntax error.

If you encounter this error in a pyi file that you did not create yourself, please file a bug.

python-compiler-error

The Python code contains a syntax error.

recursion-error

A recursive definition was found in a pyi file. Example:

class A(B): ...
class B(A): ...

If you encounter this error in a pyi file that you did not create yourself, please file a bug.

redundant-function-type-comment

Using both inline annotations and a type comment to annotate the same function is not allowed. Example:

def f() -> None:
  # type: () -> None  # redundant-function-type-comment
  pass

redundant-match

A pattern match over an enum covered the same case more than once.

from enum import Enum
class Color(Enum):
  RED = 0
  GREEN = 1
  BLUE = 2

def f(x: Color):
  match x:
    case Color.RED:
      return 10
    case Color.GREEN:
      return 20
    case Color.RED | Color.BLUE:  # redundant-match
      return 20

reveal-type

The error message displays the type of the expression passed to it. Example:

import os
reveal_type(os.path.join("hello", u"world"))  # reveal-type: unicode

This feature is implemented as an error to ensure that reveal_type() calls are removed after debugging.

signature-mismatch

The overriding method signature doesn’t match the overridden method:

class A:
  def f(self, x: int) -> None:
    pass

class B(A):
  def f(self, x:int, y: int) -> None:  # signature-mismatch
    pass
class A:
  def f(self, x: int) -> None:
    pass

class B(A):
  def f(self, x:int, y: int = 0) -> None:
    pass

See FAQ on why it can cause problems.

typed-dict-error

A TypedDict has been accessed with an invalid key. Example:

from typing import TypedDict

class A(TypedDict):
  x: int
  y: str

a = A()
a['z'] = 10

unbound-type-param

This error currently applies only to pyi files. The type parameter is not bound to a class or function. Example:

from typing import AnyStr
x = ...  # type: AnyStr  # unbound-type-param

Unbound type parameters are meaningless as types. If you want to take advantage of types specified by a type parameter’s constraints or bound, specify those directly. So the above example should be rewritten as:

from typing import Union
x = ...  # type: Union[str,  unicode]

unsupported-operands

A binary operator was called with incompatible arguments. Example:

x = "hello" ^ "world"  # unsupported-operands

wrong-arg-count

The function was called with the wrong number of arguments. Example:

def add(x, y):
  return x + y
add(1, 2, 3)  # wrong-arg-count

If you believe you are seeing this error due to a bug on pytype’s end, see this section for where the type information we use is located.

wrong-arg-types

The function was called with the wrong argument types. Example:

def f(x: int):
  pass
f(42.0)  # wrong-arg-types

If you are seeing a Non-Iterable String Error, please see FAQ.

If you believe you are seeing this error due to a bug on pytype’s end, see this section for where the type information we use is located.

wrong-keyword-args

The function was called with the wrong keyword arguments. Example:

def f(x=True):
  pass
f(y=False)  # wrong-keyword-args

If you believe you are seeing this error due to a bug on pytype’s end, see this section for where the type information we use is located.