[Python-3000] PEP 3113 (Removal of Tuple Parameter Unpacking) (original) (raw)

Jim Jewett jimjjewett at gmail.com
Sun Mar 4 02:57:39 CET 2007

I have mixed feelings; I won't go so far as to say I oppose removing tuple-arguments, but some of the problems do have other solutions.

On 3/2/07, Brett Cannon <brett at python.org> wrote:

But there is great difficulty when it comes to tuple parameters. The existence of a tuple parameter is denoted by it name being made of a . and a number in the covarnames attribute of the function's code object.

I think this is just an implementation detail. For example, co_varnames could itself hold tuples as well as names. (That would itself break backwards compatibility, but only for introspection.)

Consider PEP 3102 (keyword-only arguments) and PEP 3107 (function annotations) [#pep-3102] [#pep-3107]. Both PEPs have been accepted and introduce new functionality within a function's signature. And yet for both PEPs the new feature cannot be applied to tuple parameters.

I hadn't realized that they couldn't be annotated. That could be fixed.

On the other hand, the existence of annotations does take away part of the value -- now you could just annotate a_b_c as having internal structure: (a, (b, c))

I'm not sure what keyword-only could even mean for strictly positional arguments.

The existence of tuple parameters also places sequence objects separately from mapping objects in a function signature. There is no way to pass in a mapping object (e.g., a dict) as a parameter and have it unpack in the same fashion as a sequence does into a tuple parameter.

Either **kwargs or f(**mapping) is pretty close, depending on your perspective.

The main difference is that sequence users have to supply all the names (keys) in advance, so you can't default only part of a tuple. (This oddness does strengthen the "but it really represents a unitary something" aspect.)

Iterating over every line in Python's code repository in the Lib/ directory using the regular expression ^\s*def\s*\w+\s*\( to detect function and method definitions there were 22,252 matches in the trunk.

Tacking on .*,\s*\( to find def statements that contained a tuple parameter, only 41 matches were found. This means that for def statements, only 0.18% of them seem to use a tuple parameter.

That misses first-argument tuples, such as cgitb.text and pydoc.fixup. Searching '^\sdef\s\w+\s\(\s\(' in C:\Python25\Lib*.py ...

I got 19 first-argument hits, but ten were in test, and another 4 in site-packages.

I think it would also miss multiline defs whose tuple argument was not on the first line.

That said, the frequency is almost certainly still less than 0.5%.

-jJ

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