How can I use "number placeholders" when using .replace()?

Question

I have a file which has multiple instances of "position": [num, num] (num is a random number).

Now I want to replace those instances with "position": [num, num, 0], so basically just add a , 0 after the two numbers. But how can I do that?

I tried doing text.replace('"position": [%d, %d]', '"position": [%d, %d, 0]'). That didn't work though.

You probably want a _regular expression_, so you can replace based on the pattern rather than the exact text. Read https://docs.python.org/3/library/re.html, https://stackoverflow.com/q/4736/3001761. — jonrsharpe, Jan 02 '23 at 22:56
@KlasŠ. that won't work, because there are also other strings that use "]" in the file... — LiQ, Jan 02 '23 at 23:01

ljmc · Accepted Answer · 2023-01-02T23:40:43.467

2

You can use the following re.sub to do the replacements (demo).

Edit for OP's comments:

Given that the file is fairly small, I'd iterate through it while reading, applying the substitution as I go, and then dump all the subbed lines into the same location.

import re

file_path = "/tmp/74987707.txt"  # substitute your own

output_lines = []

with open(file_path, mode="rt") as f:
    for line in f:
        subbed_line = re.sub(
            r'("position": \[\d+(?:\.?\d*), \d+(?:\.?\d*))(])',
            r'\1, 0\2',
            line
            )
        output_lines.append(subbed_line)

with open(file_path, mode="wt") as f:
    f.writelines(output_lines)

Regex explanation:

group 1 ("position": \[\d+(?:\.?\d*), \d+(?:\.?\d*))

( start
"position": \[ match exactly (escaping [)
\d+ match one or more digit
(?:\.?\d*) non capturing group, match a possible literal dot . and one or more digits
, match exactly
\d+(?:\.?\d*) see above
) end

group 2 (\])

( start
\] match exactly (escaping ])
) end

edited Jan 02 '23 at 23:40

answered Jan 02 '23 at 23:06

ljmc

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An explanation of the regex would be good (I know regex101 gives you an explanation, but what if that link [rots?](https://en.wikipedia.org/wiki/Link_rot)) – Pranav Hosangadi Jan 02 '23 at 23:09
Fair enough, adding it now. – ljmc Jan 02 '23 at 23:11
Where can I insert the string and what should I put for line, when the string is on only one line? – LiQ Jan 02 '23 at 23:22
Do you intend to read the file and update it in place ? And how big is that file ? – ljmc Jan 02 '23 at 23:23
@ljmc Yes. The file is 1463 KB big. – LiQ Jan 02 '23 at 23:27

score 1 · Answer 2 · answered Jan 02 '23 at 23:07

From the structure of what you posted it looks like a json file, you can just loop through the items and append 0

import json

with open('example.json', 'r') as f:
    data = json.load(f)

for item in data:
    if 'position' in item:
        if isinstance(item['position'], list) and len(item['position']) == 2:
            item['position'].append(0)

with open('example_modified.json', 'w') as f:
    json.dump(data, f)

How can I use "number placeholders" when using .replace()?

2 Answers2