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PythonTricks.md

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Alphanumeric elements only

import re

re.sub(r'\W+', '', string)
  • Sub all non-alphanumeric elements \W with ''

Find most common element in list

import statistics 
from statistics import mode 

List = [2, 1, 2, 2, 1, 3] 
print(mode(List))

# 2

Convert String to List

list(string)

Iterate through 2 lists at once

a = [1, 3, 5]
b = [2, 4, 6]

for num1, num2 in zip(a, b):
    print(num1),
    print(num2)
# 1 2 3 4 5 6

Stacks

.append()

  • Add new elements to the top of the stack

.pop()

  • Remove elements in LIFO order

List implementation

  • Speed issues once large
  • lists are stored to provide fast access to random elements
  • Elements stored next to each other (Python knows exactly where to look in memory to find it)
  • Amortized constant time - if the stack grows bigger than the block of memory, Python does memory allocations

Deque implementation

From collections import deque

theStack = deque()
  • Implemented with Double Linked Lists
  • Constant time addition + removal of entries
  • Random indexing is slower, but that is not what we use a stack for

Peeking a deque

q = deque()
q.append(1)
q.append(2)
q.append(3)

q[0] # 1
q[-1] # 3
  • Just like an array

Slice Notation

a[start:stop]  # items start through stop-1
a[start:]      # items start through the rest of the array
a[:stop]       # items from the beginning through stop-1
a[:]           # a copy of the whole array
a[start:stop:step] # start through not past stop, by step
a[slice(start, stop, step)] # equivalent slice code
  • :stop is not inclusive!
a[-1]    # last item in the array
a[-2:]   # last two items in the array
a[:-2]   # everything except the last two items
a[::-1]    # all items in the array, reversed
a[1::-1]   # the first two items, reversed
a[:-3:-1]  # the last two items, reversed
a[-3::-1]  # everything except the last two items, reversed
  • These values can also be negative

Swap 2 elements

arr[i], arr[j] = arr[j], arr[i]
  • Assigns arr[i] = arr[j] and arr[j] = arr[i] all in 1 go
  • The comma forms a tuple

Destructuring with lists/tuples:

a, b = [1, 2] # a = 1, b = 2
a, b = (1, 2) # a = 1, b = 2 (comma operator above)

def fx():
  return (1, 2, 3)  # () optional, just add readability for tuples

a, b, c = fx()  # a = 1; b = 2; c = 3