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Author terry.reedy
Recipients docs@python, terry.reedy
Date 2011-05-26.21:33:09
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One of most common recurring topics on python-list (perhaps monthly) is newbies tripping over a mutation method returning None when they expect the collection. Today's example: "Puzzled by list-appending behavior".
An excerpt from the responses: 

"On 5/26/2011 11:58 AM, MRAB wrote:
> On 26/05/2011 06:17, Chris Rebert wrote:

>> list.remove(), list.sort(), and list.extend() similarly return None
>> rather than the now-modified list.

> I'd just like to point out that it's a convention, not a rigid rule.
> Sometimes it's not followed, for example, dict.setdefault."

In Python 1, I believe it was true that all mutation methods of mutable builtin collections (ie, lists -- I do not think dicts had any and sets did not exist yet) returned None. With the introduction of list.pop, the return-None rule was 'broken', though the meme continues. However, the flip side of that rule, do-not-return-the-collection, continues to be true. And the return_None rule was not really broken, but broadened to "return None or an item from the collection". I believe this should be better documented than it is now.


1. State the general rule.

A. Tutorial: At the top of chapter 5, just after "This chapter describes some things you’ve learned about already in more detail, and adds some new things as well.", add something like

"For lists, sets, and dicts, methods that change the contents or order never return the instance. Instead, they return an item from the instance or, more commonly, None."

B. Library Manual: Near the top of 4. Built-in Types, after "The principal built-in types are numerics, sequences, mappings, classes, instances and exceptions.", add something like

"Some collection classes are mutable. The methods that add, subtract, or rearrange their members never return the collection instance itself but instead return an item from the instance or None."

Comment: This rule applies to special methods like __getitem__ and __setitem__. 'lis.append(item)' is equivalent to lis.__setitem__(len(lis):len(lis), item), so it should not be so surprising that it has the same return.

2. Document None returns explicitly.

They are currently documented implicitly, by absence.

A. Docstrings: Compare the relevant parts of the output from 'help(list.append)' and 'help(list.pop)'

   L.append(object) -- append object to end
   L.pop([index]) -> item -- remove and return item at index 

Add ' -> None' to specify return for .append.
   L.append(object) -> None -- append object to end

I expect experienced Python programmers may object that this puts too much emphasis on the unimportant null return, but it is important that Python programmers know about it and experience shows that many newbies *need* that emphasis.

B. Doc entries: Essentially same suggestion -- add 'Return None.' to appropriate entries in tutorial chapter 5. For library manual, possibly add footnote '(9) Returns None' to the method table in 4.6.4. Mutable Sequence Types. For sets and dicts, add 'and return None' to appropriate entries.
Date User Action Args
2011-05-26 21:33:11terry.reedysetrecipients: + terry.reedy, docs@python
2011-05-26 21:33:10terry.reedysetmessageid: <>
2011-05-26 21:33:10terry.reedylinkissue12192 messages
2011-05-26 21:33:09terry.reedycreate