How to create a single-line script with two for loops, in the debugger, in python 2.7

Question

Creating a single line for loop in the python debugger or django shell is easy:

>>>> for x in (1,2,3,4):print(x);
>>>> for x in Obj.objects.all():something(x);

But how can I get a second for loop in there?

>>>> for x in (1,2,3,4):print x;for y in (5,6):print x,y;
SyntaxError: invalid syntax

I care because it's nice to have up arrow edit of the prior command, when working interactively (this is not an attempt to use single line commands in any other context).

NOTE: the "print" is just an example. In real use I'd iterate objects or perform other programming or debugging tasks such as 'for s in Section.objects.all():for j in s.children():print j'. I am using Python 2.7.

John La Rooy · Accepted Answer · 2013-10-26T07:24:09.923

6

For the times that a list comprehension just won't do

for x in (1,2,3,4):print x;exec("for y in (5,6):print x,y;")

or

for s in Section.objects.all():exec("for j in s.children():print j")

Sometimes you can use itertools.product (But there's no way to get the print x) like this

for x, y in itertools.product((1,2,3,4), (5,6)):print x,y)

edited Oct 26 '13 at 07:24

answered Oct 22 '13 at 06:31

John La Rooy

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I would advise strictly against `exec` unless you are hard-coding that part. – KGo Oct 22 '13 at 06:37
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@KaranGoel, the use case is when you're debugging and want to use the up arrow to edit and rerun the line. I think `exec` is ok here – John La Rooy Oct 22 '13 at 06:39
What I really wanted to do was more like 'for s in Section.objects.all():for j in s.children():print j' – Bryce Oct 26 '13 at 02:08
@Bryce, in that case you need the `exec` version. I added it to my answer – John La Rooy Oct 26 '13 at 07:24

score 3 · Answer 2 · edited May 23 '17 at 12:15

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List comprehension may be used to achieve what you want. What you want exactly is NOT possible.

>>> [(x, y) for x in (1, 2, 3, 4) for y in (5, 6)]
[(1, 5), (1, 6), (2, 5), (2, 6), (3, 5), (3, 6), (4, 5), (4, 6)]

Related: Single Line Nested For Loops

edited May 23 '17 at 12:15

Community

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answered Oct 22 '13 at 06:30

KGo

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But this ignores the `print x` part – John La Rooy Oct 22 '13 at 06:33
Because list comprehensions build lists with optional conditionals, you cannot print in them. – KGo Oct 22 '13 at 06:33
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`print` is a function in Python3, but once you do that it's getting uglier than using `exec` – John La Rooy Oct 22 '13 at 06:34
Ah, but this misses the point. I did not literally want to print numbers. I was specifically aiming to iterate through objects in each loop. – Bryce Oct 26 '13 at 02:04

score 0 · Answer 3 · answered Oct 26 '13 at 03:31

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You could end up putting the command first. For Example.

>>> [print("hi") for x in (1, 2, 3, 4) for y in (5, 6)]

You do end up with one problem however. Unless you want to call a function at the beginning I do not believe there is a way. For example.

>>> [doSomething(x, y) for x in (1, 2, 3, 4) for y in (5, 6)]

answered Oct 26 '13 at 03:31

KodyVanRy

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>>> [print("hi") for x in (1, 2, 3, 4) for y in (5, 6)] SyntaxError: invalid syntax – Bryce Oct 26 '13 at 04:37
Are you using python 3.0 or higher? I just tried this exact same thing using Python 2.7.4 and it did raise a SyntaxError on the `print` function, but then when I tried it in Python 3.3.1 it Worked perfectly fine. – KodyVanRy Oct 26 '13 at 16:12
I'm at the Python 2.7 level, and will likely be for a long time. – Bryce Oct 27 '13 at 01:53

How to create a single-line script with two for loops, in the debugger, in python 2.7

3 Answers3