Python Tips # 1

These tips are entirely based on the Python official tutorial guide.

• Using else in for statements:

You can use the else clause at the end of a for loop if you want to trigger an action as soon as the loop finishes. It will not be triggered by break:

  for num in range(2, 10):
    for x in range(2, n):
      if n % x == 0:
        print(n, 'equals', x, '*', n//x)
            break
      else:
        # loop fell through without finding a factor
        print(n, 'is a prime number')

Source

• Using simple copy vs shallow copy vs deep copy:

  • Simple copy or = will only create a reference of the object:

  old_list = [[1, 1, 1], [2, 2, 2], [3, 3, 3]]
  new_list = old_list
  new_list[2][2] = 9

  print('old_list:', old_list)
  print('id of old_list:', id(old_list))

  print('new_list:', new_list)
  print('id of new_list:', id(new_list))

  Old List: [[1, 1, 1], [2, 2, 2], [3, 3, 9]]
  ID of Old List: 140673303268168

  New List: [[1, 1, 1], [2, 2, 2], [3, 3, 9]]
  ID of New List: 140673303268168

• Shallow copy or object[:]will create a new object but will reference the nested objects:
  

  import copy
  
  old_list = [[1, 1, 1], [2, 2, 2], [3, 3, 3]]
  new_list = old_list[:] # or copy.copy(old_list)
  old_list[1][1] = 'AA'
  
  print('old_list:', old_list)
  print('id of old_list:', id(old_list))
  
  print('new_list:', new_list)
  print('id of new_list:', id(new_list))
  

```
old_list: [[1, 1, 1], [2, 'AA', 2], [3, 3, 3]]
id of old_list: 140673303268168

new_list: [[1, 1, 1], [2, 'AA', 2], [3, 3, 3]]
id of new_list: 140673303268169
```

• Finally, a deep copy will create a new object and will create new objects for the content as well:
  

  import copy
  
  old_list = [[1, 1, 1], [2, 2, 2], [3, 3, 3]]
  new_list = copy.deepcopy(old_list)
  old_list[1][1] = 'AA'
  
  print('old_list:', old_list)
  print('id of old_list:', id(old_list))
  
  print('new_list:', new_list)
  print('id of new_list:', id(new_list))
  

```
old_list: [[1, 1, 1], [2, 'AA', 2], [3, 3, 3]]
id of old_list: 140673303268168

new_list: [[1, 1, 1], [2, 2, 2], [3, 3, 3]]
id of new_list: 140673303268169
```

[Source](https://www.programiz.com/python-programming/shallow-deep-copy)

• Default values in functions are evaluated only once:

def f(a, L=[]):
    L.append(a)
    return L

print(f(1))
print(f(2))
print(f(3))

[1]
[1, 2]
[1, 2, 3]

If you don’t want the default to be shared between subsequent calls, you can write the function like this instead:

def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

Source

• Use deque to implement double-ended queues:

"Deques are a generalization of stacks and queues (the name is pronounced “deck” and is short for “double-ended queue”). Deques support thread-safe, memory efficient appends and pops from either side of the deque with approximately the same O(1) performance in either direction".

  from collections import deque
  d = deque('ghi')
  d.append('j')
  d.appendleft('f')
  j = d.pop()
  f = d.popleft()
  for elem in d:
    print(elem.upper())

Source