Mailman 3 [issue32288] Inconsistent behavior with slice assignment? - docs

Dec. 12, 2017

      New submission from Massimiliano Culpo <massimiliano.culpo@googlemail.com>:

I am not really sure if what I am reporting is to be considered a bug in the interpreter (slice built-in), a lack in documentation or if it is just me not finding the proper references (in which case feel free to close this and sorry for the noise).

Anyhow, while trying to code a class that mimics built-in lists, I noticed a few seemingly inconsistent behaviors in the use of slices to set items.

This can be reduced down to this very simple example:
```
Python 3.6.2 (default, Jul 29 2017, 00:00:00) 
[GCC 4.8.4] on linux

# 1. Slice has start and stop, step is defaulted to None
...
...
...
a = [1, 2, 3, 4]
a[1:3] = [11, 22, 33, 44]
a
[1, 11, 22, 33, 44, 4]
# 2. Same as above setting explicitly step == 1
...
...
...
a = [1, 2, 3, 4]
a[1:3:1] = [11, 22, 33, 44]
a
[1, 11, 22, 33, 44, 4]
# 3. Trying to do the same thing with step == -1
# (I naively expected either [1, 44, 33, 22, 11, 4] or 2. to fail like 3.)
...
...
...
a = [1, 2, 3, 4]
a[3:1:-1] = [11, 22, 33, 44]
Traceback (most recent call last):
  File "<input>", line 1, in <module>
ValueError: attempt to assign sequence of size 4 to extended slice of size 2

Now, the first issue: I tried to search what an "extended slice" is exactly, but found nothing in the docs. What is written here

https://docs.python.org/3/library/functions.html#slice

seems to suggest that an "extended slice" is a slice where `step is not None`, but then I would expect case 2. to behave the same as case 3. So to me it appears as either lack of documentation on the behavior, or a bug in 2.

Second issue: in the same page in the docs it's written

> slice objects are also generated when extended indexing syntax is used. For example: a[start:stop:step] or a[start:stop, i]

I wasn't aware of the second syntax, and tried to use it:

...
...
...
a = [1, 2, 3, 4]
a[1:3:1]
[2, 3]
a[1:3, 1]
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: list indices must be integers or slices, not tuple
I guess that's not the result one would expect...

Third issue: slightly related, but here

https://docs.python.org/3/library/stdtypes.html#mutable-sequence-types

I see that the documentation of `insert` reads:

> inserts x into s at the index given by i (same as s[i:i] = [x])

and from the interpreter:

help(list.insert)
Help on method_descriptor:
insert(...)
    L.insert(index, object) -- insert object before index

```

Here the fact is that `insert` really behaves like `s[i:i] = [x]`, but in my understanding it doesn't keep the promise to `insert object before index`.
```
# "Out of bounds" on the right
...
...
...
a = [1, 2, 3, 4]
a.insert(101, 11)
a
[1, 2, 3, 4, 11]
# "Out of bounds" on the left
...
...
...
a.insert(-101, 11)
a
[11, 1, 2, 3, 4]
I don't know honestly if docs are not clear here, or if it is just me, but I didn't expect this behavior and thought it was worth reporting.

Let me know if you need more information.

----------
assignee: docs@python
components: Documentation, Interpreter Core
messages: 308120
nosy: Massimiliano Culpo, docs@python
priority: normal
severity: normal
status: open
title: Inconsistent behavior with slice assignment?
type: behavior
versions: Python 2.7, Python 3.4, Python 3.5, Python 3.6

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Python tracker &lt;report@bugs.python.org>
&lt;https://bugs.python.org/issue32288>
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[issue32288] Inconsistent behavior with slice assignment?

Massimiliano Culpo

Steven D'Aprano

Stefan Krah

Stefan Krah

Massimiliano Culpo

Stefan Krah

R. David Murray

Stefan Krah

Serhiy Storchaka

Massimiliano Culpo

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participants (5)