Convert a list with strings all to lowercase or uppercase

I have a Python list variable that contains strings. Is there a function that can convert all the strings in one pass to lowercase and vice versa, uppercase?

It can be done with list comprehensions

>>> [x.lower() for x in ["A", "B", "C"]]
['a', 'b', 'c']
>>> [x.upper() for x in ["a", "b", "c"]]
['A', 'B', 'C']

or with the map function

>>> list(map(lambda x: x.lower(), ["A", "B", "C"]))
['a', 'b', 'c']
>>> list(map(lambda x: x.upper(), ["a", "b", "c"]))
['A', 'B', 'C']

Besides being easier to read (for many people), list comprehensions win the speed race, too:

$ python2.6 -m timeit '[x.lower() for x in ["A","B","C"]]'
1000000 loops, best of 3: 1.03 usec per loop
$ python2.6 -m timeit '[x.upper() for x in ["a","b","c"]]'
1000000 loops, best of 3: 1.04 usec per loop

$ python2.6 -m timeit 'map(str.lower,["A","B","C"])'
1000000 loops, best of 3: 1.44 usec per loop
$ python2.6 -m timeit 'map(str.upper,["a","b","c"])'
1000000 loops, best of 3: 1.44 usec per loop

$ python2.6 -m timeit 'map(lambda x:x.lower(),["A","B","C"])'
1000000 loops, best of 3: 1.87 usec per loop
$ python2.6 -m timeit 'map(lambda x:x.upper(),["a","b","c"])'
1000000 loops, best of 3: 1.87 usec per loop

>>> list(map(str.lower,["A","B","C"]))
['a', 'b', 'c']

List comprehension is how I'd do it, it's the "Pythonic" way. The following transcript shows how to convert a list to all upper case then back to lower:

pax@paxbox7:~$ python3
Python 3.5.2 (default, Nov 17 2016, 17:05:23) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> x = ["one", "two", "three"] ; x
['one', 'two', 'three']

>>> x = [element.upper() for element in x] ; x
['ONE', 'TWO', 'THREE']

>>> x = [element.lower() for element in x] ; x
['one', 'two', 'three']

For this sample the comprehension is fastest

$ python -m timeit -s 's=["one","two","three"]*1000' '[x.upper for x in s]'
1000 loops, best of 3: 809 usec per loop

$ python -m timeit -s 's=["one","two","three"]*1000' 'map(str.upper,s)'
1000 loops, best of 3: 1.12 msec per loop
$ python -m timeit -s 's=["one","two","three"]*1000' 'map(lambda x:x.upper(),s)'
1000 loops, best of 3: 1.77 msec per loop

mylist = ['Mixed Case One', 'Mixed Case Two', 'Mixed Three']
print(list(map(lambda x: x.lower(), mylist)))
print(list(map(lambda x: x.upper(), mylist)))

a student asking, another student with the same problem answering :))

fruits=['orange', 'grape', 'kiwi', 'apple', 'mango', 'fig', 'lemon']
newList = []
for fruit in fruits:
    newList.append(fruit.upper())
print(newList)

A much simpler version of the top answer is given here by @Amorpheuses.

With a list of values in val:

valsLower = [item.lower() for item in vals]

This worked well for me with an f = open() text source.

Solution:

>>> s = []
>>> p = ['This', 'That', 'There', 'is', 'apple']
>>> [s.append(i.lower()) if not i.islower() else s.append(i) for i in p]
>>> s
>>> ['this', 'that', 'there', 'is','apple']

This solution will create a separate list containing the lowercase items, regardless of their original case. If the original case is upper then the list s will contain lowercase of the respective item in list p. If the original case of the list item is already lowercase in list p then the list s will retain the item's case and keep it in lowercase. Now you can use list s instead of list p.

If your purpose is to matching with another string by converting in one pass, you can use str.casefold() as well.

This is useful when you have non-ascii characters and matching with ascii versions(eg: maße vs masse).Though str.lower or str.upper fails in such cases, str.casefold() will pass. This is available in Python 3 and the idea is discussed in detail with the answer https://stackoverflow.com/a/31599276/4848659.

>>>str="Hello World";
>>>print(str.lower());
hello world
>>>print(str.upper());
HELLO WOLRD
>>>print(str.casefold());
hello world

If you are trying to convert all string to lowercase in the list, You can use pandas :

import pandas as pd

data = ['Study', 'Insights']

pd_d = list(pd.Series(data).str.lower())

output:

['study', 'insights']

You could try using:

my_list = ['india', 'america', 'china', 'korea']

def capitalize_list(item):
    return item.upper()

print(list(map(capitalize_list, my_list)))

Here's another solution to the problem, but I don't recommend using it. Just putting it here for completion of this topic since this solution wasn't added before.

import timeit

def foo1():
    L = ["A", "B", "C", "&"]
    return [x.lower() for x in L]
def foo2():
    L = ["A", "B", "C", "&"]
    return "%".join(L).lower().split("%")

for i in range(10):
    print("foo1", timeit.timeit(foo1, number=100000))
    print("foo2", timeit.timeit(foo2, number=100000), end="\n\n")

-

foo1 0.0814619
foo2 0.058695300000000006

foo1 0.08401910000000004
foo2 0.06001100000000004

foo1 0.08252670000000001
foo2 0.0601641

foo1 0.08721100000000004
foo2 0.06254229999999994

foo1 0.08776279999999992
foo2 0.05946070000000003

foo1 0.08383590000000007
foo2 0.05982449999999995

foo1 0.08354679999999992
foo2 0.05930219999999997

foo1 0.08526650000000013
foo2 0.060690699999999875

foo1 0.09940110000000013
foo2 0.08484609999999981

foo1 0.09921800000000003
foo2 0.06182889999999985