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bpo-33190: elucidated types.new_class docstring #6317

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bpo-33190: elucidated types.new_class docstring #6317

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22 changes: 21 additions & 1 deletion Lib/types.py
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Expand Up @@ -60,7 +60,27 @@ def _m(self): pass

# Provide a PEP 3115 compliant mechanism for class creation
def new_class(name, bases=(), kwds=None, exec_body=None):
"""Create a class object dynamically using the appropriate metaclass."""
"""Create a class object dynamically using the appropriate metaclass.

Arguments:
name -- the new class name
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@ilevkivskyi ilevkivskyi Mar 31, 2018

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Please indent the argument list.

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@Ricyteach Ricyteach Mar 31, 2018

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I tried to do it just like in PEP 257 which doesn't have indentation. But I'll indent next time.

bases -- sequence of parent classes (default ())
kwds -- mapping of keyword arguments to be supplied to parents' `__init_subclass__`
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@ilevkivskyi ilevkivskyi Mar 31, 2018

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It is not only passed to __init_subclass__, it will be passed also to __prepare__ of the computed metaclass, to the metaclass call itself, and in future to any other place required by the class creation machinery. This sentence should be reformulated to something simpler but correct, like mapping of keyword arguments in an equivalent class statement (see example below).

exec_body -- a callback used to populate the freshly created class namespace

Note: the class-creation functionality of `new_class` is similar to `type`. However,
calls to `type` cannot be directly substituted by calls to `new_class`. For example:
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@ilevkivskyi ilevkivskyi Mar 31, 2018

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Although, you were hit but this particular problem, I don't think it deserves a dedicated note. Just describing clearly every argument is enough. Plus maybe an example of usage like this:

For example, the class definition::

    class C(A, B, foo=bar):
        x = 42

can be translated to the following call to `new_class`::

    C = new_class('C', (A, B), {'foo': bar}, lambda ns: ns.update(x=42))


type("MyClass", (), dict(my_method=lambda self: None))

...is not equivalent to the below, which will produce a `TypeError`:

new_class("MyClass", (), dict(my_method=lambda self: None))

Instead, the correct way to use `new_class` in this instance is:

new_class("MyClass", (), {}, lambda ns: ns.update(dict(my_method=lambda self: None)))
"""
resolved_bases = resolve_bases(bases)
meta, ns, kwds = prepare_class(name, resolved_bases, kwds)
if exec_body is not None:
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