Python - List Comprehension
List Comprehension
List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list.
Example:
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)
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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)
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The Syntax
newlist = [expression for item in iterable if condition == True]
The return value is a new list, leaving the old list unchanged.
Condition
The condition is like a filter that only accepts the items that valuate to
True.
Example
Only accept items that are not "apple":
newlist = [x for x in fruits if x != "apple"]
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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:
Iterable
The iterable can be any iterable object, like a list, tuple, set etc.
Example
You can use the range() function to create an iterable:
newlist = [x for x in range(10)]
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Same example, but with a condition:
Example
Accept only numbers lower than 5:
newlist = [x for x in range(10) if x < 5]
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Expression
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:
Example
Set the values in the new list to upper case:
newlist = [x.upper()
for x in fruits]
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You can set the outcome to whatever you like:
Example
Set all values in the new list to 'hello':
newlist = ['hello' for x in fruits]
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The expression can also contain conditions, not like a filter, but as a way to manipulate the outcome:
Example
Return "orange" instead of "banana":
newlist = [x if x != "banana" else "orange"
for x in fruits]
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The expression in the example above says:
"Return the item if it is not banana, if it is banana return orange".