Allow file to be used as a contract type

[textbook]

Allow file to be used as a contract type within docstrings/text contracts, for example:

@contract
def file_test(in_):
    """Ensure that the file contract works.

   :type in_: file

    """
    return in_.read() 

This should resolve #36.

[AndreaCensi]

I think it could be way simpler than that:

def check_is_file(x):
    return isinstance(x, ...)

new_contract('file', check_is_file)

[textbook]

It certainly makes the code much shorter! But then instead of

ContractNotRespected: Breach for argument 'in_' to file_test().
Expected a file, got 'str'.
checking: file   for value: Instance of str: 'str' 

the user gets:

ContractNotRespected: Breach for argument 'in_' to file_test().
Value does not pass criteria of check_is_file()() (module: ...).
checking: callable()   for value: Instance of str: 'str'   
checking: file    for value: Instance of str: 'str'   
Variables bound in inner context:
- args: Instance of tuple: () 
- kwargs: Instance of dict: {}

I did look into putting it into miscellaneous_aliases, if you think that would be preferable, but it could mean rewriting ist to handle tuples of types for isinstance. Let me know what you think.

[AndreaCensi]

Fair enough --- the error messages are quite cryptic at the moment.

On Tue, Jun 2, 2015 at 8:00 PM, Jonathan Sharpe notifications@github.com wrote:

It certainly makes the code much shorter! But then instead of

ContractNotRespected: Breach for argument 'in_' to file_test(). Expected a file, got 'str'. checking: file for value: Instance of str: 'str'

the user gets

ContractNotRespected: Breach for argument 'in_' to file_test(). Value does not pass criteria of check_is_file()() (module: ...). checking: callable() for value: Instance of str: 'str' checking: filelike for value: Instance of str: 'str' Variables bound in inner context:

• args: Instance of tuple: ()
• kwargs: Instance of dict: {}

I did look into putting it into miscellaneous_aliases, if you think that would be preferable, but it could mean rewriting ist to handle tuples of types for isinstance. Let me know what you think.

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Andrea Censi | LIDS / MIT | http://censi.mit.edu