New submission from Patrick Reader <[email protected]>:
When unpacking a collection or string literal inside another literal, the
compiler should optimise the unpacking away and store the resultant collection
simply as another constant tuple, so that `[*'123', '4', '5']` is the exact
same as `['1', '2', '3', '4', '5']`.
Compare:
```
>>> dis.dis("[*'123', '4', '5']")
1 0 BUILD_LIST 0
2 BUILD_LIST 0
4 LOAD_CONST 0 ('123')
6 LIST_EXTEND 1
8 LIST_EXTEND 1
10 LOAD_CONST 1 ('4')
12 LIST_APPEND 1
14 LOAD_CONST 2 ('5')
16 LIST_APPEND 1
```
vs.
```
>>> dis.dis("['1', '2', '3', '4', '5']")
1 0 BUILD_LIST 0
2 LOAD_CONST 0 (('1', '2', '3', '4', '5'))
4 LIST_EXTEND 1
```
and `timeit` shows the latter to be over 3 times as fast.
For example, when generating a list of characters, it is easier and more
readable to do `alphabet = [*"abcde"]` instead of `alphabet = ["a", "b", "c",
"d", "e"]`. The programmer can do what is most obvious without worrying about
performance, because the compiler can do it itself.
----------
components: Interpreter Core
messages: 383837
nosy: pxeger
priority: normal
severity: normal
status: open
title: Unpacking of literals inside other literals should be optimised away by
the compiler
_______________________________________
Python tracker <[email protected]>
<https://bugs.python.org/issue42754>
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