def expect_element(__funcname=_qualified_name, **named):
"""
Preprocessing decorator that verifies inputs are elements of some
expected collection.
Examples
--------
>>> @expect_element(x=('a', 'b'))
... def foo(x):
... return x.upper()
...
>>> foo('a')
'A'
>>> foo('b')
'B'
>>> foo('c') # doctest: +NORMALIZE_WHITESPACE +ELLIPSIS
Traceback (most recent call last):
...
ValueError: ...foo() expected a value in ('a', 'b') for argument 'x',
but got 'c' instead.
Notes
-----
A special argument, __funcname, can be provided as a string to override the
function name shown in error messages. This is most often used on __init__
or __new__ methods to make errors refer to the class name instead of the
function name.
This uses the `in` operator (__contains__) to make the containment check.
This allows us to use any custom container as long as the object supports
the container protocol.
"""
def _expect_element(collection):
if isinstance(collection, (set, frozenset)):
# Special case the error message for set and frozen set to make it
# less verbose.
collection_for_error_message = tuple(sorted(collection))
else:
collection_for_error_message = collection
template = (
"%(funcname)s() expected a value in {collection} "
"for argument '%(argname)s', but got %(actual)s instead."
).format(collection=collection_for_error_message)
return make_check(
ValueError,
template,
complement(op.contains(collection)),
repr,
funcname=__funcname,
)
return preprocess(**valmap(_expect_element, named))
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