Make Strings In List Uppercase - Python 3

Question

I'm in the process of learning python and with a practical example I've come across a problem I cant seem to find the solution for. The error I get with the following code is 'list' object has to attribute 'upper'.

def to_upper(oldList):
    newList = []
    newList.append(oldList.upper())

words = ['stone', 'cloud', 'dream', 'sky']
words2 = (to_upper(words))
print (words2)

Solution 1

Since the upper() method is defined for string only and not for list, you should iterate over the list and uppercase each string in the list like this:

def to_upper(oldList):
    newList = []
    for element in oldList:
        newList.append(element.upper())
    return newList

This will solve the issue with your code, however there are shorter/more compact version if you want to capitalize an array of string.

• map function map(f, iterable). In this case your code will look like this:

  words = ['stone', 'cloud', 'dream', 'sky']
  words2 = list(map(str.upper, words))
  print (words2)
  

• List comprehension [func(i) for i in iterable].In this case your code will look like this:

  words = ['stone', 'cloud', 'dream', 'sky']
  words2 = [w.upper() for w in words]
  print (words2)
  

Solution 2

You can use the list comprehension notation and apply theupper method to each string in words:

words = ['stone', 'cloud', 'dream', 'sky']
words2 = [w.upper() for w in words]

Or alternatively use map to apply the function:

words2 = list(map(str.upper, words))