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Python check arguments types

Decorators help us wrap some routines at function invocation. Here I show a small example that raises TypeError exceptions when given args have unexpected type. Note that it is not pythonic to type check.

This recipe is quite old, as its first pieces appear in PEP-0318 in 2003. A module exists too but it looks neglected...

The (heretic) decorator itself !

def accepts(*argstypes, **kwargstypes):
    def wrapper(func):
        def wrapped(*args, **kwargs):
            if len(args) > len(argstypes):
                raise TypeError("%s() takes at most %s non-keyword arguments (%s given)" % (func.__name__, len(argstypes), len(args)))
            argspairs = zip(args, argstypes)
            for k,v in kwargs.items():
                if k not in kwargstypes:
                    raise TypeError("Unexpected keyword argument '%s' for %s()" % (k, func.__name__))
                argspairs.append((v, kwargstypes[k]))
            for param, expected in argspairs:
                if param is not None and not isinstance(param, expected):
                    raise TypeError("Parameter '%s' is not %s" % (param, expected.__name__))
            return func(*args, **kwargs)
        return wrapped
    return wrapper

Let us decorate !

>>> @accepts(str, arg2=int)
... def f(arg1, arg2=None):
...     pass
...

See it in action...

>>> f('foo')
>>> f('foo', arg2=3)
>>> f()
TypeError: f() takes at least 1 argument (0 given)
>>> f('foo', 'bar')
TypeError: f() takes at most 1 non-keyword arguments (2 given)
>>> f('foo', arg2='bar')
TypeError: Parameter 'bar' is not int
>>> f('foo', arg3='bar')
TypeError: Unexpected keyword argument 'arg3' for f()

Or with classes...

>>> class A(object):
...     pass
...
>>> class B(object):
...     @accepts(object, (str, unicode))
...     def f(self, s):
...         pass
...
...     @accepts(object, A)
...     def g(self, a):
...         pass

>>>
>>> b = B()
>>> b.f(u'foo')
>>> b.f('foo')
>>> b.g(A())
>>> b.g(B())
TypeError: Parameter '<__main__.B object at 0x902466c>' is not A

The same can be applied to return :)

def returns(rtype):
    def wrapper(f):
        def wrapped(*args, **kwargs):
            result = f(*args, **kwargs)
            if not isinstance(result, rtype):
                raise TypeError("return value %r does not match %s" % (result,rtype))
            return result
        return wrapped
    return wrapper
>>> @accepts(str, arg2=int)
... @returns(int)
... def f(arg1, arg2=None):
...     return 0
...

..but kids, don't do this at home :-)

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