Flexibly check that the result of os.path.join is of a given type.

path_join_check.py

"""
Wrapper call to os.path.join and os.path.expanduser that checks whether the 
returned path is of a given type.

Author: awan1
Date: 20151029

"""
import os

def path_join_check(a, *p, **kwargs):
    """
    Wrapper to os.path.join and os.path.expanduser, raising an error if the
    resultant path is not of a specified type.
    The expected type of the path is specified in kwargs via the named argument
    'typ'.

    Parameters:
    a, *p: arguments to os.path.join

    kwargs:
    typ: the expected type of path. Valid inputs specified by valid_typs. If typ
         is 'any', checks that the path matches one of the valid types. typ must
         be passed in.
         (Use kwargs to preserve initial syntax of os.path.join)

    Returns:
    Result of os.path.expanduser(os.path.join(...))
    Raises error if resultant path is not of type specified by typ.

    """
    # The typ value for which to check all valid typs
    any_typ_name = 'any'
    
    # Structure of typs_and_checks:
    # keys: valid typs
    # values: tuples of (the checking operation to perform, the return condition)
    typs_and_checks = {'dir': (os.path.isdir, True),
                       'file': (os.path.isfile, True)}
    valid_typs = typs_and_checks.keys()

    try:
        typ = kwargs['typ']
    except KeyError:
        raise Exception("path_join_check expects a keyword argument typ")

    if typ == any_typ_name:
        any_typ = True
    else:
        any_typ = False
        if typ not in valid_typs:
            raise Exception("typ {} not supported.".format(typ))

    path = os.path.expanduser(os.path.join(a, *p))

    for valid_typ, (check_op, ret_cond) in typs_and_checks.iteritems():
        if any_typ or typ == valid_typ:
            if check_op(path) == ret_cond: return path

    # If we reach here, we didn't get a good path.
    raise Exception("{} is not of type {}".format(path, typ))