Python basic cheatsheet

Pythonian code

General

• Conditional or Ternary operator

  result = x if a > b else y

• lambda function

  y = lambda x: x+2

• Single line functions in python are lambda functions

  perms = lambda xs: (list(x) for x in itertools.permutations(xs))  

• Ignore output by using an underscore

  _ = foo()

• splat operator: allows lists and dictionaries to be unpacked¹

    # this sends the list as inputs to the function
    def somefun(p1, p2, p3)
    a = ['a', 3, 4.5]
    some_func(*a)
    
    # this sends the dictionary as parameter, value pairs
    b = {'p1': 'a', 'p2': 3, 'p3': 4.5}
    some_fun(**b)

• walrus operator:

    y = z if (z:=np.mean(x)) > y else y

List

• list comprehension

  result = [function(items) for items in list]
  # can be used to make empty list of lists
  result = [[] for i in range(n)]

• multiple level list comprehension

  y = [x for xs in xss for x in xs]

• list comprehension + if statement

  [i for i, c in enumerate(a) if c in b]

• list comprehension + if/else statement

  result = [x if a > b else y for a in list]

• finding and removing an element from list

    indexes = [i for i, e in enumerate(list) if e == element]
    for r in reversed(range(len(indexes))):
      lines[indexes[r]] = []

• filtering elements in a list based on a condition

  # list comprehension with if statement works
  a = [x for x in a if x != 2]
   
  #alternative
  a = list(filter(lambda x: x!= 2, a))
   
  # necessary as the following line will only remove the first item in the list
  a.remove(2)

• swapping elements in a list

  A[j],A[k] = A[k],A[j]

Dict

• dict comprehension

  result = {k:v for k,v in zip(keys, values)}

• iterating through dict keys or/and values

  for key in mydict.keys():
      print(key)
  for val in mydict.values():
      print(val)
  for key, val in mydict.items():
      print(key, val)

• getting values from dictionary

  val = my_dict.get(key, default) # returns default if key does not exist

Tuple

• Combining two list into a list of tuples

  a = list(range(0,10))
  b = list(range(10,20))
  c = list(zip(a, b))

• Converting tuples into lists

  d = list(zip(*c))
  #d[0] == a
  #d[1] == b

Common modules

Argparse

Easy to setup command line input.

import argparse
parser = argparse.ArgumentParser(description='Program description')
 
parser.add_argument("folder", nargs='?', default=os.getcwd(), help="input folder") # optional positional argument
 
parser.add_argument("-v", "--verbose", dest = "verbose", default=False, action='store_true', help="Output info logs") # flag
 
parser.add_argument('--optional-argument', dest="opt_arg", default=[1.0,2.0,15.0], type=lambda y: [float(x) for x in y.split(',')], help="Input desired floats separated by ','; e.g. '1,1,1' or '12.35, 3.74, 1' ")
# optional argument that is processed by a function
 
args = parser.parse_args()

Itertools

Generating combinations in python.

Use case: Get all possible combinations of each of the lists in this list of lists² - [[1,2], [2,3], [4,5,6], [1]]

from itertools import product
source = [[1,2], [2,3], [4,5,6], [1]]
combinations = list(product(*source))

Logging

import logging
logging.basicConfig(format='%(asctime)s %(message)s',
                filename='logs.log',
                filemode='w', ## if logs need to be rewritten
                level=logging.DEBUG if debug else logging.INFO)
logging.info("Info message")
logging.debug("Debug message")

I like to copy the logs to stderr³ when a verbose flag is passed, so that I can directly read the command line output and don’t have to look at the log.

if verbose:
	logging.getLogger().addHandler(logging.StreamHandler())

os and glob

• Finding specific files in a folder:

  import os
  files = [f for f in os.listdir(folder) if f.endswith('.ext')]

  import glob
  files = glob(os.path.join(folder, "*.ext"))

Pickle

import pickle
#or 
import dill as pickle
 
with open(pickle_file, 'wb') as f:
	# Pickle the 'data' dictionary using the highest protocol available.
	pickle.dump(data, f, pickle.HIGHEST_PROTOCOL)

Note: If you have to pickle lambda functions or functions returning another function, use dill instead of pickle (drop in replacement).⁴

Regular expressions

Regular expression cheatsheet

IPython (Jupyter)

Widget list

• Autoreload libraries⁵

  %reload_ext autoreload
  %autoreload 2

• timeit - very useful to calculate how fast your function/code is

  %timeit myfunction()

• Clear variables

  %reset -f

• Progress bar

  from ipywidgets import IntProgress 
  fp = IntProgress(min=min, max=max, description='Progress')
  display(fp) # required to show the progress bar
  # update value using
  fp.value = new_value
  # Use FloatProgress if the value is float

Footnotes

1. https://docs.python.org/3/tutorial/controlflow.html#unpacking-argument-lists and https://stackoverflow.com/questions/4979542/python-use-list-as-function-parameters ↩

2. https://stackoverflow.com/questions/3169825/generating-combinations-in-python and https://docs.python.org/3/library/itertools.html#itertools.product ↩

3. https://stackoverflow.com/a/13733863 ↩

4. https://stackoverflow.com/questions/25348532/can-python-pickle-lambda-functions ↩

5. https://stackoverflow.com/questions/56059651/how-to-make-vscode-auto-reload-external-py-modules#57245926 ↩