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List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list.
Based on a list of fruits, you want a new list, containing only the fruits with the letter "a" in the name. Without list comprehension you will have to write a for statement with a conditional test inside:
Example fruits = ["apple", "banana", "cherry", "kiwi", "mango"] newlist = [] for x in fruits: if "a" in x: newlist.append(x)
print(newlist)With list comprehension you can do all that with only one line of code:
Example fruits = ["apple", "banana", "cherry", "kiwi", "mango"] newlist = [x for x in fruits if "a" in x]
print(newlist)
The Syntax newlist = [expression for item in iterable if condition == True]
The return value is a new list, leaving the old list unchanged.The condition is like a filter that only accepts the items that evaluate to True.
Only accept items that are not "apple": newlist = [x for x in fruits if x != "apple"] The condition if x != "apple"
will return
True for all elements other than "apple", making the new list contain all fruits except "apple".The condition is optional and can be omitted:
newlist = [x for x in fruits]
The iterable can be any iterable object, like a list, tuple, set etc.
You can use the range() function to create an iterable: newlist = [x for x in range(10)] Same example, but with a condition:
Accept only numbers lower than 5:
Formula
newlist = [x for x in range(10) if x < 5]The expression is the current item in the iteration, but it is also the outcome, which you can manipulate before it ends up like a list item in the new list:
Set the values in the new list to upper case: newlist = [x.upper() for x in fruits] You can set the outcome to whatever you like:
Set all values in the new list to 'hello': newlist = ['hello' for x in fruits] The expression can also contain conditions, not like a filter, but as a way to manipulate the outcome:
Return "orange" instead of "banana":
newlist = [x if x != "banana" else "orange"
for x in fruits]The expression in the example above says:
"Return the item if it is not banana, if it is banana return orange".