Working with text

Goal: join, measure, tidy, and take apart strings.

Why this matters

Most of the data a program handles is text: names, messages, file contents, web pages. Python treats text as a first-class citizen, and a handful of tools will cover almost everything you need.

Joining and repeating

You met + for numbers. On strings, + joins them end to end. * repeats a string:

concat.py

first = "Ada"
last = "Lovelace"

full = first + " " + last
print(full)

print("=" * 20)
print("ha" * 3)

Ada Lovelace
====================
hahaha

Joining is called concatenation. Notice the " " in the middle: Python does not add spaces for you. first + last would give AdaLovelace.

TypeError: can only concatenate str (not "int") to str

You tried "Age: " + 36. Python will not silently turn the number into text. Either convert it with str(36), or better, use an f-string: f"Age: {age}". F-strings handle the conversion for you and are almost always the cleaner choice.

How long is it?

len() returns the number of characters in a string:

length.py

word = "Python"
sentence = "Hello, World!"

print(len(word))
print(len(sentence))
print(len(""))

6
13
0

Spaces and punctuation count. The empty string "" has length 0.

String methods

Strings come with built-in actions called methods. You call a method by writing a dot after the value, then the method name, then parentheses:

methods.py

name = "  ada lovelace  "

print(name.upper())
print(name.lower())
print(name.strip())
print(name.strip().title())
print(name.replace("ada", "Ada"))
print(name)

  ADA LOVELACE  
  ada lovelace  
ada lovelace
Ada Lovelace
  Ada lovelace  
  ada lovelace  

Method	What it does
.upper()	all capital letters
.lower()	all lowercase letters
.strip()	remove spaces from both ends
.title()	capitalize the first letter of each word
.replace(old, new)	swap every old for new

Two things to notice.

Methods can be chained. name.strip().title() strips first, then title-cases the result. Read chains left to right.

Methods do not change the original. The last line prints name, and it is still " ada lovelace ". Strings in Python are immutable: once created, they never change. A method hands you a new string, and if you want to keep it, you store it:

name = name.strip()

Cleaning up input

People type stray spaces and random capitals. input("Name: ").strip() is a habit worth forming now. And answer.lower() == "yes" accepts Yes, YES, and yes all at once.

Discovering more methods

Strings have dozens of methods. You do not need to memorize them. In Visual Studio Code, type a string variable followed by a dot, and a list pops up. Two more that are worth knowing today: .startswith("...") and .endswith("..."), which return True or False.

Picking out characters

Every character in a string has a position, called its index. Counting starts at 0, not 1. Square brackets pull out the character at an index:

indexing.py

word = "Python"

print(word[0])
print(word[1])
print(word[5])
print(word[-1])
print(word[0:3])
print(word[3:])
print(word[:2])

P
y
n
n
Pyt
hon
Py

Here is "Python" with its indexes:

 P   y   t   h   o   n
 0   1   2   3   4   5
-6  -5  -4  -3  -2  -1

• word[0] is the first character, word[5] is the last of a six-letter word.
• Negative indexes count from the end: word[-1] is always the last character, whatever the length.
• word[0:3] is a slice: characters from index 0 up to but not including index 3. That gives three characters.
• Leave out the start to mean "from the beginning": word[:2]. Leave out the end to mean "to the end": word[3:].

The "up to but not including" rule feels odd at first, but it has a nice property: word[:3] + word[3:] is always the whole word.

IndexError: string index out of range

You asked for an index that does not exist, such as word[6] on a six-letter word. Remember the last index is len(word) - 1. Slices are more forgiving: word[0:100] simply stops at the end.

Try it

Store your full name in a variable. Print it in capitals, print how many characters it has, and print just the first three letters.

Common mistakes

AttributeError: 'str' object has no attribute 'Upper'

Method names are lowercase: .upper(), not .Upper().

The method did nothing

You wrote name.strip() on its own line and did not store the result. Use name = name.strip().

TypeError: 'str' object is not callable

You wrote len with square brackets or a method without parentheses, or reused len or str as a variable name. Avoid naming your variables after Python's built-in functions.

Exercises

1. Shout. Ask for a sentence and print it in capitals with three exclamation marks at the end.
2. Character count. Ask for some text and print two numbers: its length including spaces, and its length without spaces. .replace() can remove the spaces.
3. Initials. Ask for a first name and a last name. Print the initials in the form A.L., in capitals, even if the user typed in lowercase with extra spaces.

Solution 1

sentence = input("Type a sentence: ")
print(sentence.upper() + "!!!")

Solution 2

text = input("Type something: ")
cleaned = text.replace(" ", "")

print(f"Characters including spaces: {len(text)}")
print(f"Characters without spaces: {len(cleaned)}")

Replacing a space with the empty string "" deletes it.

Solution 3

first = input("First name: ").strip()
last = input("Last name: ").strip()

initials = first[0].upper() + "." + last[0].upper() + "."
print(f"Your initials are {initials}")

.strip() runs first so that a name typed as " ada " still has a at index 0.

Summary

• + joins strings, * repeats them. Use f-strings to mix text and numbers.
• len(text) counts characters.
• Methods are called with a dot: text.upper(), text.strip(), text.replace(old, new). They return a new string and leave the original alone.
• Indexes start at 0. text[-1] is the last character.
• Slices text[start:end] include start and stop just before end.

Next: Searching and checking text