I just came across this line of python:
order.messages = {c.Code:[] for c in child_orders} I have no idea what it is doing, other than it is looping over the list child_orders and placing the result in order.messages.
What does it do and what is it called?
That's a dict comprehension.
It is just like a list comprehension
[3*x for x in range(5)] --> [0,3,6,9,12] except:
{x:(3*x) for x in range(5)} ---> { 0:0, 1:3, 2:6, 3:9, 4:12 } dictionary, not a list{} not square braces []In your case the keys are coming from the Code property of each element and the value is always set to empty array []
The code you posted:
order.messages = {c.Code:[] for c in child_orders} is equivalent to this code:
order.messages = {} for c in child_orders: order.messages[c.Code] = [] See also:
It's dictionary comprehension!
It's iterating through child_orders and creating a dictionary where the key is c.Code and the value is [].
More info here.
Just like list comprehension in Python, it's called dictionary comprehension.
sample_list = [2,4,6,8,9,10] dict = {val: val**2 for val in sample_list if val**2 % 2 == 0} print(dict) //Output: {8: 64, 2: 4, 4: 16, 10: 100, 6: 36} The code snippet above maps the numbers to their squares that are even numbers.
Just in addition, and since the question's title is very general: some people ending here might also be looking for the set comprehension as it also contains curly braces and a for loop. So a set comprehension would look like
order.messages = {c.Code for c in child_orders} and it will return a Python set. In contrast to the former dictionary comprehension which will return a dictionary:
order.messages = {c.Code:[] for c in child_orders} 