[Python-ideas] New 3.x restriction on number of keyword arguments (original) (raw)

M.-A. Lemburg mal at egenix.com
Thu Oct 21 17:41:12 CEST 2010

Georg Brandl wrote:

Am 21.10.2010 16:06, schrieb Benjamin Peterson:

Raymond Hettinger <raymond.hettinger at ...> writes:

One of the use cases for named tuples is to have them be automatically created from a SQL query or CSV header. Sometimes (but not often), those can have a huge number of columns. In Python 2.x, it worked just fine -- we had a test for a named tuple with 5000 fields. In Python 3.x, there is a SyntaxError when there are more than 255 fields. I'm not sure why you think this is new. It's been true from at least 2.5 as far as I can see. You must be talking of a different restriction. This snippet works fine in 2.7, but raises a SyntaxError in 3.1: exec("def f(" + ", ".join("a%d" % i for i in range(1000)) + "): pass")

The AST code in 2.7 raises this error for function/method calls only. In 3.2, it also raises the error for function/method definitions.

Looking at the AST code, the limitation appears somewhat arbitrary. There's no comment in the code suggesting a reason for the limit and it's still possible to pass in more arguments via *args and **kws - but without the built-in argument checking.

Could someone provide some insight ?

Note that it's not uncommon to have more than 255 possible function/method arguments in generated code, e.g. in database abstraction layers.

Thanks,

Marc-Andre Lemburg eGenix.com

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