Python tricks

in Python
#Python Tricks

1) Iterate over a list pairwise

Let l be a list containing n elements. We want to run through it pairwise : 0/1, 1/2, ..., (n-1)/n

e.g. l = [0,1,2,3,4,5,6,7,8,9] -> 0/1, 1/2, 2/3, ..., 8/9

for a, b in zip(l[:-1], l[1:]):
    print(f'a = {a}, b = {b}')

or

from itertools import tee

def pairwise(iterable):
    a, b = tee(iterable)
    next(b, None)
    return zip(a, b)

for a, b in pairwise(l):
    print(f'a = {a}, b = {b}')

The latter is a bit faster than the former.

Time comparison:

l = [x for x in range(1000000)]

Algorithm Naive Naive (pre-computed limit) zip() pairwise()
Time 0.46799 0.33899 0.07999 0.07700

2) Quickly rename files in a folder

In the next snippets, 'OLD' is the old name of the file, and 'NEW', well, the new one.

import os
[os.rename(f, f.replace('OLD', 'NEW')) for f in os.listdir('PATH_TO_FOLDER')]

You can add a condition, like a string found in the file name:

import os
[os.rename(f, f.replace('OLD', 'NEW')) for f in os.listdir('PATH_TO_FOLDER') if 'STRING_TO_FIND' in f]

Un-rolled version:

import os

for f in os.listdir('PATH_TO_FOLDER'):
    if 'STRING_TO_FIND' in f:
        os.rename(f, f.replace('OLD', 'NEW'))

3) Quickly rename files in a folder and all subfolders

import os
for root, dirs, files in os.walk(r'PATH_TO_FOLDER'):
    for i in files:
        if 'STRING_TO_FIND' in i:
            print(os.path.join(root, i))
            os.rename(os.path.join(root, i), os.path.join(root, i.replace('OLD', 'NEW')))

4) Find a rogue print-statement

This will print the file and the line where it comes from.

import sys
import traceback

class TracePrints(object):
  def __init__(self):
    self.stdout = sys.stdout
  def write(self, s):
    self.stdout.write("Writing %r\n" % s)
    traceback.print_stack(file=self.stdout)

sys.stdout = TracePrints()