For Loop questions

Question

I'm used to seeing For Loops in this format:

for number in l: sum = sum + number

I was browsing some forums and came across this piece of code:

count_chars = ".arPZ"
string = "Phillip S. is doing a really good job."
counts = tuple(string.count(d) for(d) in count_chars)
print counts

I'm not sure if that is really a For loop, so I decided to rewrite it in a way that I understood:

tuple(
for(d) in count_chars:
 string.count(d))

Needless to say, it failed lol. So can someone explain what is going on, and explain the folly of my logic? Thanks!!

• anon582847382: This is called a generator expression. I am sure someone will post a more expert answer than I will write.
• dyao: Thanks for getting me in the right direction. I had no idea what a generator expression was!
• Gareth Latty: Here is a short video about list comprehensions and it's siblings, including generator expressions.
• anon582847382: @user3386440 Nobody posted anything (correct) so I wrote something up, see below.
• Ffisegydd: possible duplicate of Python `for` syntax: block code vs single line generator expressions

Answer 1

It's not quite a for loop as such, but a generator expression. What it basically does is return an iterator where each element is the amount of time every character in count_chars occurs in d. It then adds all of these elements into a tuple.

It is (roughly) equivalent to:

counts = []
for d in count_chars:
 counts.append(string.count(d))
counts = tuple(counts)

• anon582847382: @user3386440 declare an empty list counts. For every element in count_chars, count how many times it is in string and add that number to the list counts. Convert counts to a tuple.
• dyao: Thanks again. I'll look up append on my own but I think I got the rest of it. Assume I didn't want it to appear on a list ie: (2,2,1,1,0). Instead I want it to be be simply print out. I will show you want I mean in the OP.