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FAQ

How is pytype different from other type checkers?

pytype has the ability to infer types for unannotated code. For more information, check out:

Can I find out what pytype thinks the type of my expression is?

Yes, insert reveal_type(expr) as a statement inside your code. This will cause pytype to emit an error that will describe the type of expr.

How do I reference a type from within its definition? (Forward References)

To reference a type from within its definition (e.g. when a method's return type is an instance of the class to which the method belongs), specify the type as a string. This will be resolved later by PyType. For example:

class Person(object):
  def CreatePerson(name: str) -> 'Person':
    ...

I'm dynamically populating a class / module using setattr or by modifying locals() / globals(). Now pytype complains about missing attributes or module members. How do I fix this?

Add _HAS_DYNAMIC_ATTRIBUTES = True to your class or module.

Why didn't pytype catch that my program (might) pass an invalid argument to a function?

pytype accepts a function call if there's at least one argument combination that works. For example,

def f(x: float):
  return x
f(random.random() or 'foo')

is not considered an error, because f() works for float. I.e., the str argument isn't considered. (This will change at some point in the future.) Note that this is different to attribute checking, where e.g.

(random.random() or 'foo').as_integer_ratio()

will indeed result in a type error.

Why didn't pytype catch that I changed the type of an annotated variable?

pytype uses an annotation as the initial type of a variable but does not protect against the type from changing. For example, the following patterns are allowed:

x: str = ""
x = 0

and

x: List[str] = []
x.append(0)

We will disallow both of these patterns at some point in the future.

How do I declare that something can be either byte string or unicode?

Using typing.Text if it is conceptually a text object, typing.Union[bytes, typing.Text] otherwise. See the style guide for more information.

I'm trying to use a mixin, but pytype raises errors about it. What should I do?

This happens when a mixin expects the classes it is mixed into to define particular functions. Let's say we have a LoggerMixin class that expects a name method to be used in the log message:

class LoggerMixin:
  ...  # Other initialization.
  def log(self, msg: str):
    self._log.print(f'{self.name()}: {msg}')

When pytype checks LoggerMixin, it will raise an error that LoggerMixin has no method name. The solution is to make the mixin class have all the methods it needs.

One way to do this is to create an abstract base class that defines the expected API for the mixin:

import abc
class LoggerMixinInterface(metaclass=abc.ABCMeta):
  @abc.abstractmethod
  def name(self) -> str:
    raise NotImplementedError

class LoggerMixin(LoggerMixinInterface):
  ...  # Other initialization
  def log(self, msg: str):
    self._log.print(f'{self.name()}: {msg}')

class Person(LoggerMixinInterface):
  ...  # Other initialization
  def name(self):
    return self._name

With this setup, pytype won't complain about LoggerMixin.name, and it's clear that LoggerMixin should only be mixed into classes that implement name.

Why is pytype taking so long?

If pytype is taking a long time on a file, the easiest workaround is to disable it with a skip-file directive. Otherwise, there are a few things you can try to speed up the analysis:

  • Split up the file. Anecdotally, pytype gets noticeable slower once a file grows past ~1500 lines.
  • Annotate the return types of functions to speed up inference.
  • Simplify function inputs (e.g., by reducing the number of types in unions) to speed up checking.
  • Avoid large concrete data structures (e.g., a module-level dict of a hundred constants). pytype tracks individual values for some builtin data structures, which can quickly get unwieldy.

How do I disable all pytype checks for a particular file?

You can use

# pytype: skip-file

at the start of the file to disable all checking for a particular file, while still checking the rest of the blaze target that includes it.

How do I disable all pytype checks for a particular import?

You can use

from typing import Any
import foo  # type: Any

to disable checking for module foo. Note that pytype will still verify that foo is present among your target's dependencies. To disable that check as well, replace # type: Any with # type: ignore.

How do I write code that is seen by pytype but ignored at runtime?

You can nest it inside an if typing.TYPE_CHECKING: block. This is occasionally needed to, for instance, conditionally import a module that is only used to provide type annotations.

Note that regardless of whether you use TYPE_CHECKING, if you're using a build system, you'll need to list all modules you import as dependencies of your target. That can lead to cycles in your build graph. Typically, that means that, short of rearranging your source tree, you won't be able to annotate with that specific type. You can typically work around the "inexpressible" type by inserting Any where you would have used it. See the style guide for more information.