python how to uppercase some characters in string

Question

Here is what I want to do but doesn't work:

mystring = "hello world"
toUpper = ['a', 'e', 'i', 'o', 'u', 'y']
array = list(mystring)

for c in array:
    if c in toUpper:
        c = c.upper()
print(array) 

"e" and "o" are not uppercase in my array.

Answer 1

You can use the str.translate() method to have Python replace characters by other characters in one step.

Use the string.maketrans() function to map lowercase characters to their uppercase targets:

try:
    # Python 2
    from string import maketrans
except ImportError:
    # Python 3 made maketrans a static method
    maketrans = str.maketrans 

vowels = 'aeiouy'
upper_map = maketrans(vowels, vowels.upper())
mystring.translate(upper_map)

This is the faster and more 'correct' way to replace certain characters in a string; you can always turn the result of mystring.translate() into a list but I strongly suspect you wanted to end up with a string in the first place.

Demo:

>>> try:
...     # Python 2
...     from string import maketrans
... except ImportError:
...     # Python 3 made maketrans a static method
...     maketrans = str.maketrans 
... 
>>> vowels = 'aeiouy'
>>> upper_map = maketrans(vowels, vowels.upper())
>>> mystring = "hello world"
>>> mystring.translate(upper_map)
'hEllO wOrld'