How to create a list of dictionaries from a csv file without list comprehension

Question

The output must be like this:

[{'id': '1', 'first_name': 'Heidie','gender': 'Female'}, {'id': '2', 'first_name': 'Adaline', 'gender': 'Female'}, {...}

There is a code snippet that works, running this requirement.

with open('./test.csv', 'r') as file_read:
   reader = csv.DictReader(file_read, skipinitialspace=True)
   listDict = [{k: v for k, v in row.items()} for row in reader]
   print(listDict)

However, i can't understand some points about this code above:

List comprehension: listDict = [{k: v for k, v in row.items()} for row in reader]
- How the python interpret this?
- How does the compiler assemble a list always with the header (id,first_name, gender) and their values?
- How would be the implementation of this code with nested for

I read theese answers, but i still do not understand:

My csv file:

id,first_name,last_name,email,gender
1,Heidie,Philimore,[email protected],Female
2,Adaline,Wapplington,[email protected],Female
3,Erin,Copland,[email protected],Female
4,Way,Buckthought,[email protected],Male
5,Adan,McComiskey,[email protected],Male
6,Kilian,Creane,[email protected],Male
7,Mandy,McManamon,[email protected],Female
8,Cherish,Futcher,[email protected],Female
9,Dave,Tosney,[email protected],Male
10,Torr,Kiebes,[email protected],Male

The comprehensions are a waste in this case. The dict comprehension is unneeded as every row already is a dict. The list comprehension is thus as well, since no further operations on elements are needed. A simple ˋlistDict = list(reader)ˋ should suffice. — MisterMiyagi, Feb 15 '19 at 06:54

NemoMeMeliorEst · Accepted Answer · 2019-02-15T06:55:31.227

your list comprehension :

listDict = [{k: v for k, v in row.items()} for row in reader]

equals:

item_list = []

#go through every row
for row in reader:
    item_dict = {}
    #in every row go through each item
    for k,v in row.items():
        #add each items k,v to dict.
        item_dict[k] = v
    #append every item_dict to item_list
    item_list.append(item_dict)

print(item_list)

EDIT (some more explanation):

#lets create a list
list_ = [x ** 2 for x in range(0,10)]
print(list_)

this returns:

[0,1,4,9,16,25,36,49,64,81]

You can write this as:

 list_ = []
 for x in range(0,10):
     list_.append(x ** 2)

So in that example yes you read it 'backwards'

Now assume the next:

#lets create a list
list_ = [x ** 2 for x in range(0,10) if x % 2 == 0]
print(list_)

this returns:

[0,4,16,36,64]

You can write this as:

 list_ = []
 for x in range(0,10):
     if x % 2 == 0:
         list_.append(x ** 2)

So thats not 100% backwards, but it should be logical whats happening. Hope this helps you!

should i always read the list comprehension codes back to front? Besides that, how the `k` identifies always the header? — Joao Albuquerque, Feb 15 '19 at 06:47
edited the post. hope its clearer. In the beginning its a 'weird' syntax, but as you get used to it, it saves you a lot of typing and actually becomes better readable ;) — NemoMeMeliorEst, Feb 15 '19 at 06:56

How to create a list of dictionaries from a csv file without list comprehension

1 Answers1