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PEP622: Be more explicit about side-effects (#1503)

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Daniel F Moisset 2020-07-07 21:47:42 +01:00 committed by GitHub
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@ -796,8 +796,8 @@ Only match-by-name will work by default, and classes should define
match-by-position. Additionally, dataclasses and named tuples will
support match-by-position out of the box. See below for more details.
Exception semantics
-------------------
Exceptions and side effects
---------------------------
While matching each case, the ``match`` statement may trigger execution of other
functions (for example ``__getitem__()``, ``__len__()`` or
@ -807,6 +807,15 @@ an ``AttributeError`` raised while trying to lookup an attribute while matching
attributes of a Class Pattern; that case results in just a matching failure,
and the rest of the statement proceeds normally.
The only side-effect carried on explicitly by the matching process is the binding of
names. However, the process relies on attribute access,
instance checks, ``len()``, equality and item access on the subject and some of
its components. It also evaluates constant value patterns and the left side of
class patterns. While none of those typically create any side-effects, some of
these objects could. This proposal intentionally leaves out any specification
of what methods are called or how many times. User code relying on that
behavior should be considered buggy.
The standard library
--------------------