PyContracts is a Python package that allows to declare constraints on function parameters and return values. It supports a basic type system, variables binding, arithmetic constraints, and has several specialized contracts (notably for Numpy arrays).

See documentation and installation instructions on GitHub.

Examples

It's very useful to check Numpy array dimensions:

from contracts import contract

@contract(a='array[MxN],M>0,N>0', b='array[NxP],P>0', returns='array[MxP]')
def matrix_multiply(a,  b):
    ''' Multiplies two matrices together with compatible dimensions. '''
    ...

You can also define new contracts:

from contracts import new_contract

new_contract('rgb', 'array[HxWx3](uint8),H>10,W>10')

A slightly more complicated example:

from contracts import contract
@contract(       a='list[ M ](type(x))',
                 b='list[ N ](type(x))',
           returns='list[M+N](type(x))')
def my_cat_equal(a, b):
    ''' Concatenate two lists together, requiring they contain
        elements of the same type. '''
    ...

An example using Python 3 annotations:

from contracts import contract

@contract
def my_function(a : 'int,>0', b : 'list[N],N>0') -> 'list[N]':
     """ Requires b to be a nonempty list, and the return
         value to have the same length. """
     ...