Should I force Python type checking?

Stop doing that.

The point of using a "dynamic" language (that is strongly typed as to values*, untyped as to variables, and late bound) is that your functions can be properly polymorphic, in that they will cope with any object which supports the interface your function relies on ("duck typing").

Python defines a number of common protocols (e.g. iterable) which different types of object may implement without being related to each other. Protocols are not per se a language feature (unlike a java interface).

The practical upshot of this is that in general, as long as you understand the types in your language, and you comment appropriately (including with docstrings, so other people also understand the types in your programme), you can generally write less code, because you don't have to code around your type system. You won't end up writing the same code for different types, just with different type declarations (even if the classes are in disjoint hierarchies), and you won't have to figure out which casts are safe and which are not, if you want to try to write just the one piece of code.

There are other languages that theoretically offer the same thing: type inferred languages. The most popular are C++ (using templates) and Haskell. In theory (and probably in practice), you can end up writing even less code, because types are resolved statically, so you won't have to write exception handlers to deal with being passed the wrong type. I find that they still require you to programme to the type system, rather than to the actual types in your programme (their type systems are theorem provers, and to be tractable, they don't analyse your whole programme). If that sounds great to you, consider using one of those languages instead of python (or ruby, smalltalk, or any variant of lisp).

Instead of type testing, in python (or any similar dynamic language) you'll want to use exceptions to catch when an object does not support a particular method. In that case, either let it go up the stack, or catch it, and raise your exception about an improper type. This type of "better to ask forgiveness than permission" coding is idiomatic python, and greatly contributes to simpler code.

* In practice. Class changes are possible in Python and Smalltalk, but rare. It's also not the same as casting in a low level language.

Update: You can use mypy to statically check your python outside of production. Annotating your code so they can check that their code is consistent lets them do that if they want; or yolo it if they want.

In most of the cases it would interfere with duck typing and with inheritance.

• Inheritance: You certainly intended to write something with the effect of

  assert isinstance(d, dict)

  to make sure that your code also works correctly with subclasses of dict. This is similar to the usage in Java, I think. But Python has something that Java has not, namely

• Duck typing: most built-in functions do not require that an object belongs to a specific class, only that it has certain member functions that behave in the right way. The for loop, e.g., does only require that the loop variable is an iterable, which means that it has the member functions __iter__() and next(), and they behave correctly.

Therefore, if you do not want to close the door to the full power of Python, do not check for specific types in your production code. (It might be useful for debugging, nevertheless.)

If you insist on adding type checking to your code, you may want to look into annotations and how they might simplify what you have to write. One of the questions on StackOverflow introduced a small, obfuscated type-checker taking advantage of annotations. Here is an example based on your question:

>>> def statictypes(a):    def b(a, b, c):        if b in a and not isinstance(c, a[b]): raise TypeError('{} should be {}, not {}'.format(b, a[b], type(c)))        return c    return __import__('functools').wraps(a)(lambda *c: b(a.__annotations__, 'return', a(*[b(a.__annotations__, *d) for d in zip(a.__code__.co_varnames, c)])))>>> @statictypesdef orSearch(d: dict, query: dict) -> type(None):    pass>>> orSearch({}, {})>>> orSearch([], {})Traceback (most recent call last):  File "<pyshell#162>", line 1, in <module>    orSearch([], {})  File "<pyshell#155>", line 5, in <lambda>    return __import__('functools').wraps(a)(lambda *c: b(a.__annotations__, 'return', a(*[b(a.__annotations__, *d) for d in zip(a.__code__.co_varnames, c)])))  File "<pyshell#155>", line 5, in <listcomp>    return __import__('functools').wraps(a)(lambda *c: b(a.__annotations__, 'return', a(*[b(a.__annotations__, *d) for d in zip(a.__code__.co_varnames, c)])))  File "<pyshell#155>", line 3, in b    if b in a and not isinstance(c, a[b]): raise TypeError('{} should be {}, not {}'.format(b, a[b], type(c)))TypeError: d should be <class 'dict'>, not <class 'list'>>>> orSearch({}, [])Traceback (most recent call last):  File "<pyshell#163>", line 1, in <module>    orSearch({}, [])  File "<pyshell#155>", line 5, in <lambda>    return __import__('functools').wraps(a)(lambda *c: b(a.__annotations__, 'return', a(*[b(a.__annotations__, *d) for d in zip(a.__code__.co_varnames, c)])))  File "<pyshell#155>", line 5, in <listcomp>    return __import__('functools').wraps(a)(lambda *c: b(a.__annotations__, 'return', a(*[b(a.__annotations__, *d) for d in zip(a.__code__.co_varnames, c)])))  File "<pyshell#155>", line 3, in b    if b in a and not isinstance(c, a[b]): raise TypeError('{} should be {}, not {}'.format(b, a[b], type(c)))TypeError: query should be <class 'dict'>, not <class 'list'>>>> 

You might look at the type-checker and wonder, "What on earth is that doing?" I decided to find out for myself and turned it into readable code. The second draft eliminated the b function (you could call it verify). The third and final draft made a few improvements and is shown down below for your use:

import functoolsdef statictypes(func):    template = '{} should be {}, not {}'    @functools.wraps(func)    def wrapper(*args):        for name, arg in zip(func.__code__.co_varnames, args):            klass = func.__annotations__.get(name, object)            if not isinstance(arg, klass):                raise TypeError(template.format(name, klass, type(arg)))        result = func(*args)        klass = func.__annotations__.get('return', object)        if not isinstance(result, klass):            raise TypeError(template.format('return', klass, type(result)))        return result    return wrapper

Edit:

It has been over four years since this answer was written, and a lot has changed in Python since that time. As a result of those changes and personal growth in the language, it seems beneficial to revisit the type-checking code and rewrite it to take advantage of new features and improved coding technique. Therefore, the following revision is provided that makes a few marginal improvements to the statictypes (now renamed static_types) function decorator.

#! /usr/bin/env python3import functoolsimport inspectdef static_types(wrapped):    def replace(obj, old, new):        return new if obj is old else obj    signature = inspect.signature(wrapped)    parameter_values = signature.parameters.values()    parameter_names = tuple(parameter.name for parameter in parameter_values)    parameter_types = tuple(        replace(parameter.annotation, parameter.empty, object)        for parameter in parameter_values    )    return_type = replace(signature.return_annotation, signature.empty, object)    @functools.wraps(wrapped)    def wrapper(*arguments):        for argument, parameter_type, parameter_name in zip(            arguments, parameter_types, parameter_names        ):            if not isinstance(argument, parameter_type):                raise TypeError(f'{parameter_name} should be of type '                                f'{parameter_type.__name__}, not '                                f'{type(argument).__name__}')        result = wrapped(*arguments)        if not isinstance(result, return_type):            raise TypeError(f'return should be of type '                            f'{return_type.__name__}, not '                            f'{type(result).__name__}')        return result    return wrapper