Hi ! What will be the output if this code 2 or 4 ? please provide a few explainantion.

Question

d = {k:v for k, v in['a1','b2']}
d['a1'] = 1
d['b2'] = 2
print(len(d))

Answer 1

the first line is a dictionary comprehension

it is the same as:

d = {}
for k,v in ['a1','b2']:
d.update({k:v})

k, v in this case will unpack each string in the list.
for the first item in the list:
k, v = 'a1'
k='a'
v='1'
for the second item in the list:
k, v = 'b2'
k = 'b'
v = '2'

so initially d would become
d = {'a':'1', 'b':'2'}

but then you add these values:
d['a1'] = 1
d['b2'] = 2

so d becomes:
d = {'a':'1', 'b':'2', 'a1':1, 'b2':2}

so what is len(d) ?

Answer 2

Youness
This code declares the variable d as a dictionary and uses a loop to assign string values from a list consisting of two strings of two characters each.
The result is a dictionary of two keys a and b with values 1 and 2.
Then the dictionary d is supplemented with two more keys a1 and b2.
As a result, the dictionary consists of four keys.
Answer: 4.

#d = {k:v for k, v in['a1','b2']}
d = {}
for k, v in ['a1','b2']:
d[k] = v

d['a1'] = 1
d['b2'] = 2
print(len(d), d)

I hope the code is clearer for you in this format.

A question for reflection:
Why didn't I assign values the way it is written in the first case?

for k, v in ['a1','b2']:
d = {k:v}

Answer 3

Have you run the code in the code playground? That might help.

Answer 4

Bob_Li while I was translating my answer into English, you beat me to it.
If I had seen your answer, I wouldn't have wasted time explaining myself...😎

Answer 5

Bob_Li oh no, I did not see this error in you, because when I wrote my answer I did not see your answer. I addressed this question to the author of the discussion, I only suggested that difficulties might arise when rewriting the teral expression...😎

Answer 6

Solo

you're right...😅

it should be d[k]=v

for k,v in ['a1','b2']:
d = {k:v}
is wrong.
it would just overwrite d at each iteration and you would just get
d = {'b':'2'}
in the end.🤪

I updated my answer with another method that can use {k:v} though...
because there must be an equivalent method to why k:v works in the dictionary comprehension. 😅

Answer 7

Solo
but my solution was initially wrong. I did not test it. my bad. You did great pointing out the mistake.👍

Answer 8

The output of the code will be:
2
The dictionary `d` is initialized using a dictionary comprehension with the key-value pairs ['a1', 'b2']. However, it is important to note that ['a1', 'b2'] is treated as an iterable of characters, not key-value pairs. Therefore, the resulting `d` dictionary will have two keys, 'a' and 'b', with corresponding values '1' and '2' respectively.

When `len(d)` is called, it returns the number of key-value pairs in the dictionary, which is 2.

Answer 9

Vashu Chaudhary but how about the two additional items added afterwards? That would make the len = 4

Answer 10

The two additional items mentioned in your question are not actually added to the dictionary because the dictionary comprehension only iterates over the existing items in the list, which are `'a1'` and `'b2'`.