# Useful functions and classes

#### Function wrappers for statements

def noop(*args, **kw_args): pass

def and_(a, b): return a and b

#### Object metadata

def type_name(value): return type(value).__name__

def module(value): return type(value).__module__.split('.')

def root_module(value): return module(value)[0]

#### Type checking

class ConstraintError(ValueError):

    def __init__(self, check_func, value):

        ValueError.__init__(self, str(value)+' must satisfy constraint '

            +check_func.__name__)

def cast(type_, val):

    '''Passes None through. Does not cast a subclass to a superclass (which

    doesn't make sense in a dynamically-typed language).'''

    if val != None and not isinstance(val, type_): val = type_(val)

    return val

def is_str(val): return isinstance(val, basestring)

def is_list(val): return isinstance(val, list)

#### Basic types

def none_if(val, none_val):

    if cast(type(none_val), val) == none_val: return None

    else: return val

#### Iterables

def first(iter_): return iter_.next()

def skip(iter_, func):

    # Advance iter while func is True

    try:

        while func(iter_.curr()): iter_.next()

    except StopIteration: pass # nothing after the matching elements

def list_subset(list_, idxs):

    subset = []

    for idx in idxs:

        try: subset.append(list_[idx])

        except IndexError: pass

    return subset

class CheckedIter:

    def __init__(self, check_func, iterable):

        self.check_func = check_func

        self.iter_ = iterable.__iter__()

    def __iter__(self): return self

    def next(self):

        entry = self.iter_.next()

        if self.check_func(entry): return entry

        else: raise ConstraintError(self.check_func, entry)

#### Lists

def list_get(list_, idx, default=None):

    try: return list_[idx]

    except IndexError: return default

def list_eq_is(list0, list1):

    '''Compares two lists using is'''

    if len(list0) != len(list1): return False

    for i in xrange(len(list0)):

        if list0[i] is not list1[i]: return False

    return True

#### Dicts

def rename_key(dict_, orig, new): dict_[new] = dict_.pop(orig)

def dict_subset(dict_, keys):

    subset = dict()

    for key in keys:

        try: subset[key] = dict_[key]

        except KeyError: pass

    return subset