[Python-ideas] New 3.x restriction on number of keyword arguments

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

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.

Marc-Andre Lemburg

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