Description

assignee = None
closed_at = None
created_at = <Date 2021-01-27.22:01:00.254>
labels = ['3.9', 'docs']
title = 'Python tutorial could make rules for default argument definition clearer'
updated_at = <Date 2021-01-27.22:01:00.254>
user = 'https://bugs.python.org/jessevsilverman'

activity = <Date 2021-01-27.22:01:00.254>
actor = 'jessevsilverman'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2021-01-27.22:01:00.254>
creator = 'jessevsilverman'
dependencies = []
files = []
hgrepos = []
issue_num = 43043
keywords = []
message_count = 1.0
messages = ['385813']
nosy_count = 2.0
nosy_names = ['docs@python', 'jessevsilverman']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue43043'
versions = ['Python 3.9']

Activity

1. jessevsilverman commented on Jan 27, 2021

   

   jessevsilvermanmannequin

   on Jan 27, 2021

   MannequinAuthor

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   I love the tutorial, but have been pointing out things I only knew from brutal experience, other tutorials, reasoning from other languages I know better, or looking things up. I only mention them if they seem in scope and at the same level of the material which is presented.

   Regarding 4.7.1. Default Argument Values of the excellent Python tutorial:

   I guessed that I would see the behavior shown below from my understanding of default arguments in several other languages. I did not read anything in this tutorial section that would lead me to conclude this, and other things mentioned in this section are neither much more or less subtle, more or less obvious, or more or less important.

   PS F:\\PythonCode\> py VarArgs.py
     File "F:\\PythonCode\\VarArgs.py", line 1
       def varArgs(quote, footnote='', times):
                                            ^
   SyntaxError: non-default argument follows default argument
   

   Most introductions to default argument definition in other languages go out of their way to highlight that the default parameters can be only the last N of them, and that all required ones must come first.
   Python is intended to be accessible to people no more clever than the other languages intend, so tho it seems obvious to me that they must only come after all required arguments, I remember when it was not obvious (and fortunately the tutorials I read in the past for other languages belabored the point).

   The explanatory text of:
   giving one of the optional arguments: ask_ok('OK to overwrite the file?', 2)

   as it merely says "one of", rather than "the first of" led me to wonder if Python was somehow more flexible/better at mind-reading than the languages I already know well that require all default args to be trailing, and to only let you default the last one, two, or ... N. The results of the experiment shown above demonstrate it is similar to the behavior in other languages in this regard.

   I deeply appreciate the information about when the default arguments get evaluated, which is highly relevant and is not something I would have picked up on from other languages I've used.

   A simple line stating that "any default arguments must follow any required arguments in the definition" and something like "on calls, optional arguments may only be omitted from the end of the parameter list, they can't simply be skipped" would be awesome.

   I am not sure whether or not this is made perfectly clear in the next section, which has a lot of other new and interesting ideas flying around in it as well, besides this one.
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12. jeff14994 commented on Sep 5, 2025

    

    jeff14994

    on Sep 5, 2025

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    Hi, I’d like to work on this issue.

    To address the concern, I propose a small wording change in section [4.9.1. Default Argument Values](https://docs.python.org/3/tutorial/controlflow.html#default-argument-values).

    Current text:

    The most useful form is to specify a default value for one or more arguments. This creates a function that can be called with fewer arguments than it is defined to allow.

    Proposed text:

    The most useful form is to specify a default value for one or more arguments. All arguments without default values must come first, followed by those with defaults. This allows a function to be called with fewer arguments than it is defined to allow.

    Would this wording change be acceptable, or would you prefer a different approach?

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