What is this python expression containing curly braces and a for in loop?

Question

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?

Answer 1

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 }

• produces a Python dictionary, not a list
• uses curly braces {} not square braces []
• defines key:value pairs based on the iteration through a list

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:

• PEP0274
• Python Dictionary Comprehension

Answer 2

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.

Answer 3

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.