comprehensions

a comprehension helps to shorten the code

produces a list

does not only work for lists only, but for any iterable

new_output = [output expression for iterator in iterable]

code line reduction

long for-code

my_list = ["ant", "bear", "cat", "dog", "eagle"]

my_new_list = []

for element in my_list:

  if "a" in element:

    my_new_list.append(element) #the output is a list with all elements with the substring "a"

short for-code

my_list = ["ant", "bear", "cat", "dog", "eagle"]

my_new_list = [element for element in my_list if "a" in element]

be aware

my_list = ["ant", "bear", "cat", "dog", "eagle"]

my_new_list = ["a" in element for element in my_list] #this code creates only True and False in the list

variants

nested comprehension

pairs = [(num1, num2) for num1 in rang(0,2) for num2 in rang(4,5)] #creates tuples of specific number pairs

matrix

print([[col for col in range(0,5)] for row in range(0,3)]) #creates a list matrix of 3x5 as [[0, 1, 2, 3, 4] , [0, 1, 2, 3, 4] , [0, 1, 2, 3, 4]]

syntax examples

my_nums = [1, 2, 3]

my_new_nums = [num + 1 for num in my_nums]

print([[col for col in range(0,5)] for row in range(0,5)]) #creates a list matrix of 5x5

produces a dictionary

syntax examples

print({num: num * (-1) for num in range(10)}) #creates a dictionary of numbers

cities = ['Hamburg', 'Berlin', 'Hannover', 'Stuttgart', 'Kiel', 'Ulm', 'Dresden']

print({city: len(city) for city in cities }) #creates a dictionary of cities and the number of letters in their name

produces a generator object

generator_object = (output expression for iterator in iterable)

generator expression will not need the memory and calculation resources of list comprehensions

generator function

generator functions do not return values, they yield values

syntax examples

my_gen_object = (num for num in range(0, 31)) #creates an object, not a list

print(list((num for num in range(0, 31)))) #outputs a list from the generator object

def num_sequence(n):

  i = 0

  while i < n:

    i += 1

    yield i

print(list(num_sequence(3))) #gives out the list 1, 2, 3.

def my_func(val):

  for i in val:

    yield i.upper() #makes upper case for all values in val