python.md

Useful commands in python

Filehandling, pickling, etc.

String operations, Regexes, etc.

Find all occurrences of a given substring in a string

import re
re.finditer(substring, string)

Sorting, datastructures, etc.

Sort a list in Python

list.sort(reverse=True|False, key=func)

func is a callback, often a lambda expression :

list.sort(reverse=True, key = lambda list_element: list_element[1]) # in a list of tuples, sort ascendingly, using the second value of each tuple as key 

Note: Sorting happens in-place

Argsort a numpy array

sort_idcs = np.argsort(similarity.tolist())

Collections

Counter

Returns a dictionary giving each element of a list as key and the number of times that element appears as a value.

from collections import Counter
dummy = ['A','A','B']
from collections import Counter
counter = Counter(dummy)
print(counter)
Counter({'A': 2, 'B': 1})

Note that min and max operation on Counter are applied to the counter keys

print(max(counter))
print(min(counter))
B
A

unless an explicit func for the key is supplied

print(max(counter, key=counter.get))
print(min(counter, key=counter.get))
A
B

Defaultdict

Very handy to avoid having to initialize lists separately when using them as values in a dictionary.

s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
d = defaultdict(list)
for k, v in s:
     d[k].append(v)

sorted(d.items())
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

For nested defaultdicts do

d = defaultdict(lambda: defaultdict(list))

Map, Filter and Reduce

Map

items = [1, 2, 3, 4, 5]
squared = list(map(lambda x: x**2, items))

Filter

number_list = range(-5, 5)
less_than_zero = list(filter(lambda x: x < 0, number_list))
print(less_than_zero)

Reduce

from functools import reduce
product = reduce((lambda x, y: x * y), [1, 2, 3, 4])

Other neat tools

tqdm for progress bars

from tqdm import tqdm

for element in tqdm(list):
    ## Do some stuff here with element

Pathlib

Create dir if it doesn't exist yet

from pathlib import Path
Path('stampscollection/monte_carlo/').mkdir(parents=True, exist_ok=True)

Iterators

Elegant way to increment a global counter

import itertools

global_counter1 = itertools.count()

next(global_counter1)

Assertions and exception handling

Assertions

assert number > 0, f"number greater than 0 expected, got: {number}"