Steven D'Aprano <steve+pyt...@pearwood.info> added the comment:
If you know about timeit, why aren't you using it?
In any case, I have just ran your nested_lists.py file, and on my
computer, the last version with zip is the fastest version:
0 transpose1_0(lT) Time = 1.0627508163452149e-05
7 zip(*lT) Time = 1.5511512756347657e-06
1.55e-6 is smaller than 1.06e-5:
py> 1.55e-6 < 1.06e-5
True
Smaller times are faster, and the first version transpose1_0 takes
nearly seven times longer to run than the last version with zip.
As for the other comments, the purpose of the tutorial is to teach the
language in the simplest way possible. It is aimed at beginners, not
intermediate or expert programmers. The purpose of this section is to
demonstrate the basic features of list comprehensions, not to overwhelm
the beginner with complicated details.
Making the example more complicated so that it is a tiny bit faster
would not a good tradeoff for the tutorial, but in fact all the more
complicated versions are slower, not faster.
You are also confused about the structure of comprehensions. The
comprehension consists of
expression-part for-part (optional for- and if-parts)
There is no else-part in the comprehension, and the for-part always
comes before any if-part. Your code:
row[j] if j < len(row) else 0
is not an "if statement" as you call it, neither is it part of the
comprehension syntax. It is the expression-part of the comprehension,
containing a ternary if-expression. It is not part of the structure of
the comprehension.
See the full language specification, in particular the grammar:
https://docs.python.org/3.9/reference/grammar.html
We could re-write the if-expression like this:
(j < len(row)) and row[j] or 0
That doesn't mean that comprehensions consist of an "and" part followed
by an "or" part followed by a for-part. The "and" and "or" are just part
of the expression part of the comprehension.
Adding this level of technical detail and complexity to an introductory
tutorial aimed at beginners would not be a good idea.
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<https://bugs.python.org/issue41590>
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