How to pythonically avoid this code duplication with a while loop?

Question

I want to find the first filename myfile????.txt that doesn't exist yet (???? is a number). This works:

import os
i = 0
f = 'myfile%04i.txt' % i
while os.path.exists(f):
    i += 1
    f = 'myfile%04i.txt' % i

but I don't like the code duplication of f = ....

Is there a pythonic way to avoid the code duplication in this while loop?

NB: I have posted a half-satisfactory solution, using a do/while idiom, as mentioned in the main answer of Emulate a do-while loop in Python?, but I still wonder if there is better way for this particular case (thus, it's not a dupe of this question).

`glob.glob` with some `regex` would probably be the best way to go. — Ma0, Nov 27 '18 at 14:35
A nice solution is coming next year: https://www.python.org/dev/peps/pep-0572/ — VPfB, Nov 27 '18 at 15:02

Chris_Rands · Accepted Answer · 2018-11-27T14:47:08.227

6

You do not need to follow the while paradiagm here, a nested generator expression with next() works:

import os
from itertools import count
f = next(f for f in ('myfile%04i.txt' % i for i in count()) if not os.path.exists(f))
print(f)

edited Nov 27 '18 at 14:47

answered Nov 27 '18 at 14:44

Chris_Rands

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Perfect solution (I just edited so that we can easily test it by copying/pasting). – Basj Nov 27 '18 at 14:46
@Basj ok, note I just edited it a bit to correct a mistake (of mine) – Chris_Rands Nov 27 '18 at 14:47
1

This is really pythonic, concise, and explicit! I thought it would be possible with a generator / next() but I didn't find how to do it, thanks a lot! – Basj Nov 27 '18 at 14:49

Bill the Lizard · Answer 2 · 2018-11-27T14:47:14.977

3

Get rid of the f variable.

import os

i = 0
while os.path.exists('myfile%04i.txt' % i):
    i += 1

edited Nov 27 '18 at 14:47

answered Nov 27 '18 at 14:35

Bill the Lizard

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Oh great! But I still need `f` at the end, is there a way to get it? – Basj Nov 27 '18 at 14:36
Make `f` a variable and "give" it to `while`. – Ctrl S Nov 27 '18 at 14:37
@Basj Yes, you could still initialize `f` before the loop, then use `while os.path.exists(f % i)` – Bill the Lizard Nov 27 '18 at 14:39
2

The second solution doesn't work, if you `print f`, you'll get `myfile%04i.txt` and not `myfile0002.txt` for example. So we would need to add `f = f % i` at the end to really get the answer in `f`. It's probably the best solution so far though! – Basj Nov 27 '18 at 14:43
Ah, you're right. My bad for not running it. I'm going to revert my edit, since that's essentially what you started with. – Bill the Lizard Nov 27 '18 at 14:47

score 0 · Answer 3 · answered Nov 27 '18 at 14:35

I nearly found the answer while writing the end of the question. After a few modifications, it works:

import os
i = 0
while True:
    f = 'myfile%04i.txt' % i
    if not os.path.exists(f):
        break
    i += 1
print f

Still I wonder if there is a more pythonic way, maybe with an iterator, generator, next(...) or anything like this.

score 0 · Answer 4 · answered Nov 27 '18 at 14:39

0

Is this too simple?

import os
f = 'myfile0000.txt'
while os.path.exists(f):
    i += 1
    f = 'myfile%04i.txt' % i

answered Nov 27 '18 at 14:39

Dominique

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It "works", like my question's code, and the solution I posted, but I wanted to know if there is a solution without duplication of `f = ...`. – Basj Nov 27 '18 at 14:41

score 0 · Answer 5 · answered Nov 27 '18 at 14:40

0

You could do:

import os
from itertools import count

cursor = count()
it = iter((path for path in map(lambda x: 'myfile%04i.txt' % x, cursor) if not os.path.exists(path)))
first = next(it, None)

if first:
    print(first)

Output

myfile0000.txt

answered Nov 27 '18 at 14:40

Dani Mesejo

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How to pythonically avoid this code duplication with a while loop?

5 Answers5