Strings

We have previously seen strings. Here we will show you quite a few more details.

A string is surrounded by single or double quotes:

name = 'Mariano'
food = "homemade lasagna is the best"

You can get the length of a string with len(), the same as the length of a list:

if __name__ == '__main__':
    name = 'Mariano'
    food = "homemade lasagna is the best"
    print(len(name))
    print(len(food))
    print(f"'{name}' has {len(name)} characters and '{food}' has {len(food)} characters.")

This will print:

7
28
'Mariano' has 7 characters and 'homemade lasagna is the best' has 28 characters.

Concatenating strings

You can use several operations with strings. If you add strings, they are concatenated. So this:

result = 'fire' + 'place'

will set result to 'fireplace'.

Often you will need to take an existing string and add on to it:

result = 'hello'
result = result + ' '
result = result + 'there'

Now result references "hello there".

You can do this more easily with +=:

result = 'hello'
result += ' '
result += 'there'

Accumulating a result with concatenation

Concatenation is particularly useful when you want to accumulate a result. For example:

def collect_words() -> str:
    result = ''
    while True:
        word = input('Enter a word: ')
        if word == '':
            break
        if result != '':
            result += ' '
        result += word

    return result


if __name__ == '__main__':
    print(collect_words())

In this code we use the result variable to accumulate all of the words a person types. We use the += to add in each word. We also use += to add a space between the words. We use one litle trick to do this. Notice:

if result != '':
    result += ' '

We don’t add the space if result is an empty string. This prevents us from adding a space at the start of result before we have concatenated any words. This prevents us from adding space the first time through the loop.

When you run this code, you might see:

Enter a word: Yer
Enter a word: a
Enter a word: wizard
Enter a word: Harry
Enter a word:
Yer a wizard Harry

Iterating over the characters in a string

You will recall that we have iterated over lists of numbers:

numbers = [1, 2, 3]
for number in numbers:
    print(number)

lists of strings:

names = ["Mario", "Maria", "Magda"]
for name in names:
    print(name)

and even lists of tuples:

players = [('John', 42), ('Emma', 33), ('Jose', 10), ('Rosaria', 18)]
for name, jersey in players:
    print(f"{name} wears jersey number {jersey}")

We can likewise iterate over the characters in a string:

line = 'what a life'
for character in line:
    print(character)

This will print out each character, including spaces, on a separate line:

w
h
a
t

a

l
i
f
e

Functions for testing strings

Following are some of the functions you can use to test strings:

• islower(): returns true if all characters are lowercase
• isupper(): returns true if all characters are uppercase
• isalpha(): true if all characters are alphabetic
• isdigit(): true if all characters are digits
• isalnum(): true if all characters are alphanumberic (alphabetic or digits)
• isspace(): true if all characters are white space (space, tab, newline)

For example:

'abc'.islower()    # True
'abC'.islower()    # False
'ABC'.isupper()    # True
'ABc'.isupper()    # False
'a'.isalpha()     # True
'ab3'.isalpha()   # False
'8'.isdigit()     # True
'89a'.isdigit()   # False
'89a'.isalnum()   # True
' \t\n'.isspace() # True

All of these functions work on variables that reference strings, whether those strings are one or many characters long. For example:

password = 'adam123'
if password.alnum():
    print('Yes')
else:
    print('No')

This will print Yes.

Capitalization

These functions will change capitalization:

• upper(): returns a new string that is all uppercase
• lower(): returns a new string that is all lowercase

For example:

'abc'.upper()                  # 'ABC'
'ABC'.lower()                  # 'abc'
'aBc'.lower()                  # 'abc'
'ABC'.lower() == 'abc'.lower() # True

No spaces

Write a function that replaces all space characters in a string with dashes.

Work with a friend to write this code. This should take only one function. You can find starter code in no_spaces.py.

Here is a solution:

def no_spaces(text: str) -> str:
    result = ''
    for c in text:
        if c.isspace():
            c = '-'
        result += c
    return result

We use the accumulate pattern to build up the result string in the variable called result. As we loop through the string, charcter by character, we change the character to be a '-' if it is a space. We then append the character to the result string.

Replacing numbers

Write a function that replaces every number in a string with a ?.

Work with a friend to write this code. This should take only one function. You can find starter code in replace_numbers.py.

Here is a solution:

def no_numbers(text: str) -> str:
    result = ''
    for char in text:
        if char.isdigit():
            result += '?'
        else:
            result += char
    return result

We again use the accumulate pattern. Here instead of changing the character to ? if it is a number, we append a ?. Notice that this means we need to use else and have two places where we accumulate into the result string.

This problem is very similar to the No spaces, and either solution could be written in either style.

Sum digits

Write a function that sums all the digits in a string.

Work with a friend to write this code. This should take only one function. You can find starter code in sum_digits.py.

Here is a solution:

def sum_digits(text: str) -> int:
    """Add all the digits found in the `text`."""
    total = 0
    for c in text:
        if c.isdigit():
            total += int(c)
    return total

This also uses the accumulate pattern. Surprise! :-) We use total to keep track of the total. And we have to use int() to convert the character to an integer that we can add into total.