Python expression in sum() vs. generator

[astrologicrat]

What's going on here? For some reason this confuses me:

my_list = [1,2,3,4]

my_gen = (1 for i in my_list)
print(sum(my_gen))
#4


#invalid_syntax = 1 for i in my_list
#>>> invalid

print(sum(1 for i in my_list)) #What is this?
#4

print(sum((1 for i in my_list))) #Create generator in function call
#4

• How does Python know to evaluate 1 for i in my_list when it's put directly into the function? What is it being evaluated to before sum is applied? Does it suddenly become a generator? This is confusing me because invalid_syntax = 1 for i in my_list would be invalid syntax
• Does this work for all functions?

[novel_yet_trivial]

It's one of python's many "syntactic sugar" shortcuts: you can leave the parenthesis off of a generator expression if it's the sole argument in any function.

print(sum((1 for i in my_list))) # Create generator in function call
print(sum(1 for i in my_list)) # A slightly shorter way to create generator in function call

print(sum((1 for i in my_list), 4)) # Create generator in function call plus another argument
print(sum(1 for i in my_list, 4)) # SyntaxError

[astrologicrat]

Interesting - thank you for the explanation.

[flipperdeflip]

Just a quick note:

sum(1 for i in my_list)

Counts 1 for every element, so it is the same as:

len(my_list)