How to use Python decorators to check function arguments?

On Python 3.3, you can use function annotations and inspect:

import inspect

def validate(f):
    def wrapper(*args):
        fname = f.__name__
        fsig = inspect.signature(f)
        vars = ', '.join('{}={}'.format(*pair) for pair in zip(fsig.parameters, args))
        params={k:v for k,v in zip(fsig.parameters, args)}
        print('wrapped call to {}({})'.format(fname, params))
        for k, v in fsig.parameters.items():
            p=params[k]
            msg='call to {}({}): {} failed {})'.format(fname, vars, k, v.annotation.__name__)
            assert v.annotation(params[k]), msg
        ret = f(*args)
        print('  returning {} with annotation: "{}"'.format(ret, fsig.return_annotation))
        return ret
    return wrapper

@validate
def xXy(x: lambda _x: 10<_x<100, y: lambda _y: isinstance(_y,float)) -> ('x times y','in X and Y units'):
    return x*y

xy = xXy(10,3)
print(xy)

If there is a validation error, prints:

AssertionError: call to xXy(x=12, y=3): y failed <lambda>)

If there is not a validation error, prints:

wrapped call to xXy({'y': 3.0, 'x': 12})
  returning 36.0 with annotation: "('x times y', 'in X and Y units')"

You can use a function rather than a lambda to get a name in the assertion failure.

To enforce string arguments to a parser that would throw cryptic errors when provided with non-string input, I wrote the following, which tries to avoid allocation and function calls:

from functools import wraps

def argtype(**decls):
    """Decorator to check argument types.

    Usage:

    @argtype(name=str, text=str)
    def parse_rule(name, text): ...
    """

    def decorator(func):
        code = func.func_code
        fname = func.func_name
        names = code.co_varnames[:code.co_argcount]

        @wraps(func)
        def decorated(*args,**kwargs):
            for argname, argtype in decls.iteritems():
                try:
                    argval = args[names.index(argname)]
                except ValueError:
                    argval = kwargs.get(argname)
                if argval is None:
                    raise TypeError("%s(...): arg '%s' is null"
                                    % (fname, argname))
                if not isinstance(argval, argtype):
                    raise TypeError("%s(...): arg '%s': type is %s, must be %s"
                                    % (fname, argname, type(argval), argtype))
            return func(*args,**kwargs)
        return decorated

    return decorator

I have a slightly improved version of @jbouwmans sollution, using python decorator module, which makes the decorator fully transparent and keeps not only signature but also docstrings in place and might be the most elegant way of using decorators

from decorator import decorator

def check_args(**decls):
    """Decorator to check argument types.

    Usage:

    @check_args(name=str, text=str)
    def parse_rule(name, text): ...
    """
    @decorator
    def wrapper(func, *args, **kwargs):
        code = func.func_code
        fname = func.func_name
        names = code.co_varnames[:code.co_argcount]
        for argname, argtype in decls.iteritems():
            try:
                argval = args[names.index(argname)]
            except IndexError:
                argval = kwargs.get(argname)
            if argval is None:
                raise TypeError("%s(...): arg '%s' is null"
                            % (fname, argname))
            if not isinstance(argval, argtype):
                raise TypeError("%s(...): arg '%s': type is %s, must be %s"
                            % (fname, argname, type(argval), argtype))
    return func(*args, **kwargs)
return wrapper

As you certainly know, it’s not pythonic to reject an argument only based on its type.
Pythonic approach is rather “try to deal with it first”
That’s why I would rather do a decorator to convert the arguments

def enforce(*types):
    def decorator(f):
        def new_f(*args, **kwds):
            #we need to convert args into something mutable   
            newargs = []        
            for (a, t) in zip(args, types):
               newargs.append( t(a)) #feel free to have more elaborated convertion
            return f(*newargs, **kwds)
        return new_f
    return decorator

This way, your function is fed with the type you expect
But if the parameter can quack like a float, it is accepted

@enforce(int, float)
def func(arg1, arg2):
    return arg1 * arg2

print (func(3, 2)) # -> 6.0
print (func('3', 2)) # -> 6.0
print (func('three', 2)) # -> ValueError: invalid literal for int() with base 10: 'three'

I use this trick (with proper conversion method) to deal with vectors.
Many methods I write expect MyVector class as it has plenty of functionalities; but sometime you just want to write

transpose ((2,4))

I think the Python 3.5 answer to this question is beartype. As explained in this post it comes with handy features. Your code would then look like this

from beartype import beartype
@beartype
def sprint(s: str) -> None:
   print(s)

and results in

>>> sprint("s")
s
>>> sprint(3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 13, in func_beartyped
TypeError: sprint() parameter s=3 not of <class 'str'>

All of these posts seem out of date – pint now provides this functionality built in. See here. Copied here for posterity:

Checking dimensionality When you want pint quantities to be used as
inputs to your functions, pint provides a wrapper to ensure units are
of correct type – or more precisely, they match the expected
dimensionality of the physical quantity.

Similar to wraps(), you can pass None to skip checking of some
parameters, but the return parameter type is not checked.

>>> mypp = ureg.check('[length]')(pendulum_period) 

In the decorator format:

>>> @ureg.check('[length]')
... def pendulum_period(length):
...     return 2*math.pi*math.sqrt(length/G)

The package typeguard provides a decorator for this, it reads the type information from type annotations, it requires Python >=3.5.2 though. I think the resulting code is quite nice.

@typeguard.typechecked
def my_function(this_var_is_an_int: int, this_var_is_a_float: float)
    ''' Here my code '''
    pass

you could try with the pydantic validation_decorator. from the documentation pydantic:

Data validation and settings management using python type annotations.
pydantic enforces type hints at runtime, and provides user friendly
errors when data is invalid.
In benchmarks pydantic is faster than all other tested libraries.

from pydantic import validate_arguments, ValidationError


@validate_arguments
def repeat(s: str, count: int, *, separator: bytes = b'') -> bytes:
    b = s.encode()
    return separator.join(b for _ in range(count))


a = repeat('hello', 3)
print(a)
#> b'hellohellohello'

b = repeat('x', '4', separator=' ')
print(b)
#> b'x x x x'

try:
    c = repeat('hello', 'wrong')
except ValidationError as exc:
    print(exc)
    """
    1 validation error for Repeat
    count
      value is not a valid integer (type=type_error.integer)
    """

For me, the codes shared above looks complicated. What I did for defining ‘generic decorator’ for type-check:

I used *args, **kwargs feature, little extra work when using function/method but easy to manage.

Appropriate example definition for test

argument_types = {
'name':str,
'count':int,
'value':float
}

Decoration Defination

//from functools import wraps

def azure_type(func):
    @wraps(func)
    def type_decorator(*args, **kwargs):
        for key, value in kwargs.items():
            if key in argument_types:
                if type(value) != argument_types[key]:
                    #enter code here
                    return 'Error Message or what ever you like to do'  
        return func(*args, **kwargs)
    return type_decorator 

Simple sample in code

// all other definitions

@azure_type
def stt(name:str, value:float)->(int):
    #some calculation and creation of int output 
    count_output = #something int
    return count_output

// call the function:

stt(name='ati', value=32.90) #can test from that