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 3.3 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.
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
ais 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
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
Any(see typeshed issue 285 for the reason).
Some imports may be silently ignored. Another source of unexpected
Anyvalues 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=skipthe same is true for modules for which a
.pyfile is found but that are not specified on the command line. (If a
.pyistub 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
import frobnicate # Error: No module "frobnicate" frobnicate.start()
You can add a
# type: ignore comment to tell mypy to ignore this
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
# type: ignore comment will only assign the implicit
type if mypy cannot find information about that particular module. So,
if we did have a stub available for
frobnicate then mypy would
# 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
You can use a simple empty list literal in a dynamically typed function (as the
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
def f() -> None: n = 1 ... n = 'x' # Type error: n has type int
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
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 f_bad(new_lst) # Fails def f_good(x: Sequence[A]) -> A: return x f_good(new_lst) # 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
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
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 that the
object type used in the above example is similar
Object in Java: it only supports operations defined for all
objects, such as equality and
isinstance(). The type
in contrast, supports all operations, even if they may fail at
runtime. The cast above would have been unnecessary if the type of
Mypy can’t infer the type of
o after the
because it only knows about
isinstance() (and the latter is better
style anyway). We can write the above code without a cast by using
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.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 >= 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
Mypy currently does not support more complex checks, and does not assign
any special meaning when assigning a
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
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
--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
--platform win32. See the documentation for
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]'
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
reveal_type calls before you can run your code.
reveal_type is always available and you don’t need to import it.
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
if TYPE_CHECKING: so that they are not executed at runtime.
from typing import List, TYPE_CHECKING if TYPE_CHECKING: import bar def listify(arg: 'bar.BarClass') -> 'List[bar.BarClass]': return [arg]
from typing import List from foo import listify class BarClass: def listifyme(self) -> 'List[BarClass]': return listify(self)
TYPE_CHECKING constant defined by the
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
at runtime. Mypy considers it to be
True when type checking.
Here’s the above example modified to use
from typing import List MYPY = False if MYPY: import bar def listify(arg: 'bar.BarClass') -> 'List[bar.BarClass]': return [arg]