Common issues

This section has examples of cases when you need to update your code to use static typing, and ideas for working around issues if mypy doesn’t work as expected. Statically typed code is often identical to normal Python code, but sometimes you need to do things slightly differently.

Can’t install mypy using pip

If installation fails, you’ve probably hit one of these issues:

  • Mypy needs Python 3.4 or later to run.
  • You may have to run pip like this: python3 -m pip install mypy.

No errors reported for obviously wrong code

There are several common reasons why obviously wrong code is not flagged as an error.

  • The function containing the error is not annotated. Functions that do not have any annotations (neither for any argument nor for the return type) are not type-checked, and even the most blatant type errors (e.g. 2 + 'a') pass silently. The solution is to add annotations.

    Example:

    def foo(a):
        return '(' + a.split() + ')'  # No error!
    

    This gives no error even though a.split() is “obviously” a list (the author probably meant a.strip()). The error is reported once you add annotations:

    def foo(a: str) -> str:
        return '(' + a.split() + ')'
    # error: Unsupported operand types for + ("str" and List[str])
    

    If you don’t know what types to add, you can use Any, but beware:

  • One of the values involved has type ``Any``. Extending the above example, if we were to leave out the annotation for a, we’d get no error:

    def foo(a) -> str:
        return '(' + a.split() + ')'  # No error!
    

    The reason is that if the type of a is unknown, the type of a.split() is also unknown, so it is inferred as having type Any, and it is no error to add a string to an Any.

    If you’re having trouble debugging such situations, reveal_type() might come in handy.

    Note that sometimes library stubs have imprecise type information, e.g. the pow() builtin returns Any (see typeshed issue 285 for the reason).

  • Some imports may be silently ignored. Another source of unexpected Any values are the “–ignore-missing-imports” and “–follow-imports=skip” flags. When you use --ignore-missing-imports, any imported module that cannot be found is silently replaced with Any. When using --follow-imports=skip the same is true for modules for which a .py file is found but that are not specified on the command line. (If a .pyi stub is found it is always processed normally, regardless of the value of --follow-imports.) To help debug the former situation (no module found at all) leave out --ignore-missing-imports; to get clarity about the latter use --follow-imports=error. You can read up about these and other useful flags in The mypy command line.

Spurious errors and locally silencing the checker

You can use a # type: ignore comment to silence the type checker on a particular line. For example, let’s say our code is using the C extension module frobnicate, and there’s no stub available. Mypy will complain about this, as it has no information about the module:

import frobnicate  # Error: No module "frobnicate"
frobnicate.start()

You can add a # type: ignore comment to tell mypy to ignore this error:

import frobnicate  # type: ignore
frobnicate.start()  # Okay!

The second line is now fine, since the ignore comment causes the name frobnicate to get an implicit Any type.

Note

The # type: ignore comment will only assign the implicit Any type if mypy cannot find information about that particular module. So, if we did have a stub available for frobnicate then mypy would ignore the # type: ignore comment and typecheck the stub as usual.

Types of empty collections

You often need to specify the type when you assign an empty list or dict to a new variable, as mentioned earlier:

a = []  # type: List[int]

Without the annotation mypy can’t always figure out the precise type of a.

You can use a simple empty list literal in a dynamically typed function (as the type of a would be implicitly Any and need not be inferred), if type of the variable has been declared or inferred before, or if you perform a simple modification operation in the same scope (such as append for a list):

a = []  # Okay because followed by append, inferred type List[int]
for i in range(n):
    a.append(i * i)

However, in more complex cases an explicit type annotation can be required (mypy will tell you this). Often the annotation can make your code easier to understand, so it doesn’t only help mypy but everybody who is reading the code!

Redefinitions with incompatible types

Each name within a function only has a single ‘declared’ type. You can reuse for loop indices etc., but if you want to use a variable with multiple types within a single function, you may need to declare it with the Any type.

def f() -> None:
    n = 1
    ...
    n = 'x'        # Type error: n has type int

Note

This limitation could be lifted in a future mypy release.

Note that you can redefine a variable with a more precise or a more concrete type. For example, you can redefine a sequence (which does not support sort()) as a list and sort it in-place:

def f(x: Sequence[int]) -> None:
    # Type of x is Sequence[int] here; we don't know the concrete type.
    x = list(x)
    # Type of x is List[int] here.
    x.sort()  # Okay!

Invariance vs covariance

Most mutable generic collections are invariant, and mypy considers all user-defined generic classes invariant by default (see Variance of generic types for motivation). This could lead to some unexpected errors when combined with type inference. For example:

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

lst = [A(), A()]  # Inferred type is List[A]
new_lst = [B(), B()]  # inferred type is List[B]
lst = new_lst  # mypy will complain about this, because List is invariant

Possible strategies in such situations are:

  • Use an explicit type annotation:

    new_lst: List[A] = [B(), B()]
    lst = new_lst  # OK
    
  • Make a copy of the right hand side:

    lst = list(new_lst) # Also OK
    
  • Use immutable collections as annotations whenever possible:

    def f_bad(x: List[A]) -> A:
        return x[0]
    f_bad(new_lst) # Fails
    
    def f_good(x: Sequence[A]) -> A:
        return x[0]
    f_good(new_lst) # OK
    

Covariant subtyping of mutable protocol members is rejected

Mypy rejects this because this is potentially unsafe. Consider this example:

from typing_extensions import Protocol

class P(Protocol):
    x: float

def fun(arg: P) -> None:
    arg.x = 3.14

class C:
    x = 42
c = C()
fun(c)  # This is not safe
c.x << 5  # Since this will fail!

To work around this problem consider whether “mutating” is actually part of a protocol. If not, then one can use a @property in the protocol definition:

from typing_extensions import Protocol

class P(Protocol):
    @property
    def x(self) -> float:
       pass

def fun(arg: P) -> None:
    ...

class C:
    x = 42
fun(C())  # OK

Declaring a supertype as variable type

Sometimes the inferred type is a subtype (subclass) of the desired type. The type inference uses the first assignment to infer the type of a name (assume here that Shape is the base class of both Circle and Triangle):

shape = Circle()    # Infer shape to be Circle
...
shape = Triangle()  # Type error: Triangle is not a Circle

You can just give an explicit type for the variable in cases such the above example:

shape = Circle() # type: Shape   # The variable s can be any Shape,
                                 # not just Circle
...
shape = Triangle()               # OK

Complex type tests

Mypy can usually infer the types correctly when using isinstance() type tests, but for other kinds of checks you may need to add an explicit type cast:

def f(o: object) -> None:
    if type(o) is int:
        o = cast(int, o)
        g(o + 1)    # This would be an error without the cast
        ...
    else:
        ...

Note

Note that the object type used in the above example is similar to Object in Java: it only supports operations defined for all objects, such as equality and isinstance(). The type Any, in contrast, supports all operations, even if they may fail at runtime. The cast above would have been unnecessary if the type of o was Any.

Mypy can’t infer the type of o after the type() check because it only knows about isinstance() (and the latter is better style anyway). We can write the above code without a cast by using isinstance():

def f(o: object) -> None:
    if isinstance(o, int):  # Mypy understands isinstance checks
        g(o + 1)        # Okay; type of o is inferred as int here
        ...

Type inference in mypy is designed to work well in common cases, to be predictable and to let the type checker give useful error messages. More powerful type inference strategies often have complex and difficult-to-predict failure modes and could result in very confusing error messages. The tradeoff is that you as a programmer sometimes have to give the type checker a little help.

Python version and system platform checks

Mypy supports the ability to perform Python version checks and platform checks (e.g. Windows vs Posix), ignoring code paths that won’t be run on the targeted Python version or platform. This allows you to more effectively typecheck code that supports multiple versions of Python or multiple operating systems.

More specifically, mypy will understand the use of sys.version_info and sys.platform checks within if/elif/else statements. For example:

import sys

# Distinguishing between different versions of Python:
if sys.version_info >= (3, 5):
    # Python 3.5+ specific definitions and imports
elif sys.version_info[0] >= 3:
    # Python 3 specific definitions and imports
else:
    # Python 2 specific definitions and imports

# Distinguishing between different operating systems:
if sys.platform.startswith("linux"):
    # Linux-specific code
elif sys.platform == "darwin":
    # Mac-specific code
elif sys.platform == "win32":
    # Windows-specific code
else:
    # Other systems

Note

Mypy currently does not support more complex checks, and does not assign any special meaning when assigning a sys.version_info or sys.platform check to a variable. This may change in future versions of mypy.

By default, mypy will use your current version of Python and your current operating system as default values for sys.version_info and sys.platform.

To target a different Python version, use the --python-version X.Y flag. For example, to verify your code typechecks if were run using Python 2, pass in --python-version 2.7 from the command line. Note that you do not need to have Python 2.7 installed to perform this check.

To target a different operating system, use the --platform PLATFORM flag. For example, to verify your code typechecks if it were run in Windows, pass in --platform win32. See the documentation for sys.platform for examples of valid platform parameters.

Displaying the type of an expression

You can use reveal_type(expr) to ask mypy to display the inferred static type of an expression. This can be useful when you don’t quite understand how mypy handles a particular piece of code. Example:

reveal_type((1, 'hello'))  # Revealed type is 'Tuple[builtins.int, builtins.str]'

Note

reveal_type is only understood by mypy and doesn’t exist in Python, if you try to run your program. You’ll have to remove any reveal_type calls before you can run your code. reveal_type is always available and you don’t need to import it.

Import cycles

An import cycle occurs where module A imports module B and module B imports module A (perhaps indirectly, e.g. A -> B -> C -> A). Sometimes in order to add type annotations you have to add extra imports to a module and those imports cause cycles that didn’t exist before. If those cycles become a problem when running your program, there’s a trick: if the import is only needed for type annotations in forward references (string literals) or comments, you can write the imports inside if TYPE_CHECKING: so that they are not executed at runtime. Example:

File foo.py:

from typing import List, TYPE_CHECKING

if TYPE_CHECKING:
    import bar

def listify(arg: 'bar.BarClass') -> 'List[bar.BarClass]':
    return [arg]

File bar.py:

from typing import List
from foo import listify

class BarClass:
    def listifyme(self) -> 'List[BarClass]':
        return listify(self)

Note

The TYPE_CHECKING constant defined by the typing module is False at runtime but True while type checking.

Python 3.5.1 doesn’t have typing.TYPE_CHECKING. An alternative is to define a constant named MYPY that has the value False at runtime. Mypy considers it to be True when type checking. Here’s the above example modified to use MYPY:

from typing import List

MYPY = False
if MYPY:
    import bar

def listify(arg: 'bar.BarClass') -> 'List[bar.BarClass]':
    return [arg]