List Comprehensions
One way to create a list with for loop:
>>> new_list = [] >>> old_list = [1, 2, 3, 4] >>> for each in old_list: ... new_list.append(each**2) ... >>> new_list [1, 4, 9, 16]
List comprehensions are an elegant way to build a list without having to use another list for loops to append values one by one.
>>> old_list = [1, 2, 3, 4] >>> new_list = [each**2 for each in old_list] >>> new_list [1, 4, 9, 16]
Multiple Assignment
>>> list = ['one', 'two', 'three'] >>> one, two, three = list >>> one 'one' >>> two 'two' >>> three 'three'
For loops
For loop over the list:
>>> list = [1,2,3,4,5] >>> for each in list: ... print(each) ... 1 2 3 4 5 >>>
I never liked i=0, i+=1 structure, but if you still need to track the index:
>>> list = [1,2,3,4,5]
>>> for i, item in enumerate(list):
... print(f'{i} - {item}')
...
0 - 1
1 - 2
2 - 3
3 - 4
4 - 5
Nested lists
>>> list = [ 1, 2, 3, [11, 22, 33, [111, 222, 333, 444]]] >>> list[3] [11, 22, 33, [111, 222, 333, 444]] >>> list[3][0] 11 >>> list[3][3] [111, 222, 333, 444] >>> list[3][3][3] 444
Sets
set – unique collection of elements – an unordered collection type. Sets are used for membership testing and eliminating duplicate entries.
my_set = {"one", "two", "three"}
- no access using an index, use for loop
- once created – cannot change items, but can add new items my_set.add(‘new item’). or .update() for multiple items
>>> list = [1,2,3,1,2,3,1,2,3,1,2,3,1,2,3]
>>> set(list)
{1, 2, 3}
>>> str = '123123123'
>>> set(str)
{'2', '1', '3'}
The membership test is faster for sets because has is implemented using a hash table.
timeit module allows measuring the execution time of small code snippets
import timeit def my_function(): pass print(timeit.timeit(my_function, number=100000)) # will run the code a set number of times
>>> import timeit
>>> timeit.timeit('3 in list', setup='list = [1, 2, 3, 4, 5, 6, 7, 8, 9];my_set=set(list)')
0.13245909999977812
>>> timeit.timeit('3 in my_set', setup='list = [1, 2, 3, 4, 5, 6, 7, 8, 9];my_set=set(list)')
0.07676899999978559
From the post about basic python terms:
# Last element
>>> l = ['aaa', 'bbb', 'ccc']
>>> l[-1]
'ccc'
# Change element with index
>>> l[0] = 'ddd'
>>> l
['ddd', 'bbb', 'ccc']
# Inserting Elements into a List
>>> l.insert(0, 'aaa')
>>> l
['aaa', 'ddd', 'bbb', 'ccc']
# Remove element by index
>>> del l[1]
>>> l
['aaa', 'bbb', 'ccc']
# Remove element with pop if need to use removed element after pop
# pop() - last element
# pop(index)
>>> popped_l=l.pop(2)
>>> l
['aaa', 'bbb']
>>> popped_l
'ccc'
# Remove element by name -> remove (only the first occurrence of the value you specify)
>>> removed_l = l.remove('aaa')
>>> l
['bbb']
# To remove all 2 from the list - while loop with remove in it:
>>> l = [1, 2, 3, 1, 2, 2, 3, 4, 5]
>>> l
[1, 2, 3, 1, 2, 2, 3, 4, 5]
>>> while 2 in l:
... l.remove(2)
...
>>> l
[1, 3, 1, 3, 4, 5]
# Sort() - sort and permanently change the list, no way to revert
# l.sort(reverse=True) - reverse alphabetical order
>>> l = ['bbb', 'aaa', 'ccc', 'ddd']
>>> l.sort()
>>> l
['aaa', 'bbb', 'ccc', 'ddd']
# Need Temporarily - sorted()
>>> l
['bbb', 'aaa', 'ccc', 'ddd']
>>> print(sorted(l))
['aaa', 'bbb', 'ccc', 'ddd']
>>> l
['bbb', 'aaa', 'ccc', 'ddd']
# Reverse Order
>>> l
['ddd', 'ccc', 'aaa', 'bbb']
>>> l.reverse()
>>> l
['bbb', 'aaa', 'ccc', 'ddd']
>>> l.reverse()
>>> l
['ddd', 'ccc', 'aaa', 'bbb']
# Range, step and even numbers
>>> even_numbers = list(range(2,11,2))
>>> print(even_numbers)
[2, 4, 6, 8, 10]
>>> min(even_numbers)
2
>>> max(even_numbers)
10
>>> sum(even_numbers)
30
#List Comprehensions
>>> squares = [value**2 for value in range(1,11)]
>>> print(squares)
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
or
>>> squares = []
>>> for value in range(1,11):
... squares.append(value**2)
...
>>> print(squares)
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
# Copy list, l = l_copy will create just var to the same list
>>> l
['ddd', 'ccc', 'aaa', 'bbb']
>>> l_copy = l[:]
>>> l
['ddd', 'ccc', 'aaa', 'bbb']
>>> l_copy
['ddd', 'ccc', 'aaa', 'bbb']
>>> l_copy.append('eee')
>>> l
['ddd', 'ccc', 'aaa', 'bbb']
>>> l_copy
['ddd', 'ccc', 'aaa', 'bbb', 'eee']
