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Beartype documentation lives at ReadTheDocs (RTD). It's readable, structured, and soothing to the deep folds of your big galactic brain. Open your mind to an ocean of mundane knowledge that will exhaust you at work. Enter... the Bearpedia:

https://beartype.readthedocs.io

The document you are now reading was once a monolithic ~316Kb file known to induce migraines in 22% of the whole devops population. For your safety, that document no longer exists. This is how much beartype cares.

Beartype is an open-source pure-Python PEP-compliant near-real-time hybrid runtime-static third-generation type checker emphasizing efficiency, usability, unsubstantiated jargon we just made up, and thrilling puns.

# Install beartype.
$ pip3 install beartype
# Edit the "{your_package}.__init__" submodule with your favourite IDE.
$ vim {your_package}/__init__.py      # <-- so, i see that you too vim

from beartype.claw import beartype_this_package       # <-- hype comes
beartype_this_package()                               # <-- hype goes

Beartype now implicitly type-checks all annotated classes, callables, and variable assignments across all submodules of your package. Congrats. This day all bugs die.

But why stop at the burning tires in only your code? Your app depends on a sprawling ghetto of other packages, modules, and services. How riddled with infectious diseases is that code? You're about to find out.

# ....................{ BIG BEAR                        }....................
# Warn about type hint violations in *OTHER* packages outside your control;
# only raise exceptions from violations in your package under your control.
# Again, at the very top of your "{your_package}.__init__" submodule:
from beartype import BeartypeConf                              # <-- this isn't your fault
from beartype.claw import beartype_all, beartype_this_package  # <-- you didn't sign up for this
beartype_this_package()                                        # <-- raise exceptions in your code
beartype_all(conf=BeartypeConf(violation_type=UserWarning))     # <-- emit warnings from other code

Beartype now implicitly type-checks all annotated classes, callables, and variable assignments across all submodules of all packages. When your package violates type safety, beartype raises an exception. When any other package violates type safety, beartype just emits a warning. The triumphal fanfare you hear is probably your userbase cheering. This is how the QA was won.

Beartype also publishes a plethora of APIs for fine-grained control over type-checking <beartype APIs>. For those who are about to QA, beartype salutes you. Would you like to know more?

# So let's do this. $ python3

# ....................{ RAISE THE PAW                   }....................
# Manually enforce type hints across individual classes and callables.
# Do this only if you want a(nother) repetitive stress injury.

# Import the @beartype decorator.
>>> from beartype import beartype      # <-- eponymous import; it's eponymous

# Annotate @beartype-decorated classes and callables with type hints.
>>> @beartype                          # <-- you too will believe in magic
... def quote_wiggum(lines: list[str]) -> None:
...     print('“{}”\n\t— Police Chief Wiggum'.format("\n ".join(lines)))

# Call those callables with valid parameters.
>>> quote_wiggum(["Okay, folks. Show's over!", " Nothing to see here. Show's…",])
“Okay, folks. Show's over!
 Nothing to see here. Show's…”
   — Police Chief Wiggum

# Call those callables with invalid parameters.
>>> quote_wiggum([b"Oh, my God! A horrible plane crash!", b"Hey, everybody! Get a load of this flaming wreckage!",])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 30, in quote_wiggum
  File "/home/springfield/beartype/lib/python3.9/site-packages/beartype/_decor/_code/_pep/_error/errormain.py", line 220, in get_beartype_violation
    raise exception_cls(
beartype.roar.BeartypeCallHintParamViolation: @beartyped
quote_wiggum() parameter lines=[b'Oh, my God! A horrible plane
crash!', b'Hey, everybody! Get a load of thi...'] violates type hint
list[str], as list item 0 value b'Oh, my God! A horrible plane crash!'
not str.

# ....................{ MAKE IT SO                      }....................
# Squash bugs by refining type hints with @beartype validators.
>>> from beartype.vale import Is  # <---- validator factory
>>> from typing import Annotated  # <---------------- if Python ≥ 3.9.0
# >>> from typing_extensions import Annotated   # <-- if Python < 3.9.0

# Validators are type hints constrained by lambda functions.
>>> ListOfStrings = Annotated[  # <----- type hint matching non-empty list of strings
...     list[str],  # <----------------- type hint matching possibly empty list of strings
...     Is[lambda lst: bool(lst)]  # <-- lambda matching non-empty object
... ]

# Annotate @beartype-decorated callables with validators.
>>> @beartype
... def quote_wiggum_safer(lines: ListOfStrings) -> None:
...     print('“{}”\n\t— Police Chief Wiggum'.format("\n ".join(lines)))

# Call those callables with invalid parameters.
>>> quote_wiggum_safer([])
beartype.roar.BeartypeCallHintParamViolation: @beartyped
quote_wiggum_safer() parameter lines=[] violates type hint
typing.Annotated[list[str], Is[lambda lst: bool(lst)]], as value []
violates validator Is[lambda lst: bool(lst)].

# ....................{ AT ANY TIME                     }....................
# Type-check anything against any type hint – anywhere at anytime.
>>> from beartype.door import (
...     is_bearable,  # <-------- like "isinstance(...)"
...     die_if_unbearable,  # <-- like "assert isinstance(...)"
... )
>>> is_bearable(['The', 'goggles', 'do', 'nothing.'], list[str])
True
>>> die_if_unbearable([0xCAFEBEEF, 0x8BADF00D], ListOfStrings)
beartype.roar.BeartypeDoorHintViolation: Object [3405692655, 2343432205]
violates type hint typing.Annotated[list[str], Is[lambda lst: bool(lst)]],
as list index 0 item 3405692655 not instance of str.

# ....................{ GO TO PLAID                     }....................
# Type-check anything in around 1µs (one millionth of a second) – including
# this list of one million 2-tuples of NumPy arrays.
>>> from beartype.door import is_bearable
>>> from numpy import array, ndarray
>>> data = [(array(i), array(i)) for i in range(1000000)]
>>> %time is_bearable(data, list[tuple[ndarray, ndarray]])
    CPU times: user 31 µs, sys: 2 µs, total: 33 µs
    Wall time: 36.7 µs
True

Beartype brings Rust- and C++-inspired zero-cost abstractions into the lawless world of dynamically-typed Python by enforcing type safety at the granular level of functions and methods against type hints standardized by the Python community in O(1) non-amortized worst-case time with negligible constant factors. If the prior sentence was unreadable jargon, see our friendly and approachable FAQ for a human-readable synopsis.

Beartype is portably implemented in Python 3, continuously stress-tested via GitHub Actions × tox × pytest × Codecov, and permissively distributed under the MIT license. Beartype has no runtime dependencies, only one test-time dependency, and only one documentation-time dependency. Beartype supports all actively developed Python versions, all Python package managers, and multiple platform-specific package managers.