for statement' without colon [duplicate]

Question

test_keys = ["Rash", "Kil", "Varsha"]
test_values = [1, 4, 5]
  
# using dictionary comprehension
# to convert lists to dictionary
res = {test_keys[i]: test_values[i] for i in range(len(test_keys))}
  
# Printing resultant dictionary 
print ("Resultant dictionary is : " +  str(res))

there should be an ending colon " : " after 'for statement' like for i in range(3) :

but this line didn't put " : " at the end of range()
res = {test_keys[i]: test_values[i] for i in range(len(test_keys))}
This is totally out of syntax i knew, how this is possible?
perhaps is it syntax for dictionary only?

Answer 1

You can do it with sets, dictionaries, lists and generators and is called set, dictionary and list comprehension respectively or generator expression:

set_comprehension = {i for i in range(10)}
dict_comprehension = {i:i for i in range(10)}
list_comprehension = [i for i in range(10)]
generator_expression = (i for i in range(10))

print(set_comprehension)
print(dict_comprehension)
print(list_comprehension)
print(generator_expression)

Output:

{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
{0: 0, 1: 1, 2: 2, 3: 3, 4: 4, 5: 5, 6: 6, 7: 7, 8: 8, 9: 9}
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
<generator object <genexpr> at 0x7fe9e8999dd0>

Answer 2

In python, the : is used to indicate that this line we enter a new bloc of code (deeper level of indentation starting next line).
Ex:

if condition:
    pass #Deeper indentation
for i in range(10):
    pass #Deeper indentation
while True:
    pass #Deeper indentation
#And many others

In list comprehension everything is on the same line so the : is not required and will cause issue since the next line don't have deeper indentation.

Answer 3

What you have described is called "dictionary comprehension" and it's an alternative syntax for creating an iterable.

It comes in useful quite often.

Similar to list comprehension:

newlist = [expression for item in iterable if condition]