I am having trouble with list comprehension in Python
Basically I have code that looks like this
output = [] for i, num in enumerate(test): loss_ = do something test_ = do something else output.append(sum(loss_*test_)/float(sum(loss_))) How can I write this using list comprehension such as:
[sum(loss_*test_)/float(sum(loss_))) for i, num in enumerate(test)] however I don't know how to assign the values of loss_ and test_
You can use a nested list comprehension to define those values:
output = [sum(loss_*test_)/float(sum(loss_)) for loss_, test_ in ((do something, do something else) for i, num in enumerate(test))] Of course, whether that's any more readable is another question.
As Yaroslav mentioned in the comments, list comprehensions don't allow you to save a value into a variable directly.
However it allows you to use functions.
I've made a very basic example (because the sample you provided is incomplete to test), but it should show how you can still execute code in a list comprehension.
def loss(): print "loss" return 1 def test(): print "test" return 5 output = [loss()*test() for i in range(10) ] print output which is this case will result in a list [5, 5, 5, 5, 5, 5, 5, 5, 5, 5]
I hope this somehow shows how you could end up with the behaviour that you were looking for.
ip_list = string.split(" ") # split the string to a list using space seperator for i in range(len(ip_list)): # len(ip_list) returns the number of items in the list - 4 # range(4) resolved to 0, 1, 2, 3 if (i % 2 == 0): ip_list[i] += "-" # if i is even number - concatenate hyphen to the current IP string else: ip_list[i] += "," # otherwize concatenate comma print("".join(ip_list)[:-1]) # "".join(ip_list) - join the list back to a string # [:-1] trim the last character of the result (the extra comma) 
c = a / float(b), do afrom __future__ import division; c = a / b, see this PEP. This is the default in Python 3.