Nick Coghlan added the comment:
Note that it's only the 3.2 interactive help that specifies "ctx=None", and
it's almost certainly lifting that directly from the Python implementation.
The C implementation is definitely in compliance with the prose docs, which
give the signature as localcontext([c]) and say "If no context is specified, a
copy of the current context is used" without indicating whether passing in None
explicitly is supposed to count as not specifying a context.
However, the specific problem that prompted this report is that you used to be
able to write code like:
def my_decimal_func(arg1, arg2, context=None):
with decimal.localcontext(context):
# perform decimal operations on arg1 and arg2
but with the C implementation, such code will break. You have to instead write
something less idiomatic like:
def my_decimal_func(arg1, arg2, *context):
with decimal.localcontext(*context):
# perform decimal operations on arg1 and arg2
Any third party Decimal manipulating function that accepts an optional context
and passes it down to the standard Decimal API will be confronted with the same
problem in 3.3: passing None as the context no longer means the same thing as
omitting the context argument entirely.
We can either handle this as a documentation issue (recommending the *args
based idiom above as an alternative to passing an explicit None), or else
change the C implementation to accept context=None as equivalent to omitting
the context argument entirely.
I'm inclined towards the latter, simply due to the backwards compatibility
breach: explicitly calling localcontext(None) would indeed be silly, but
calling localcontext(context), where context comes from a parameter with a
default value of None, is perfectly normal Python code. Idiomatic code that
worked correctly in the previous Python version is a reasonably strong argument
in favour of retaining the feature.
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue15783>
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