Slices and string methods

What you will be able to do

After this lesson you can

• take any piece out of a string with a slice,
• reverse a string, and take every second character,
• change the case of text, clean it and replace parts of it, and
• explain why a method does not change the original string.

Slices: a piece of a string

A slice takes a piece of a string. It is written with two numbers and a colon inside square brackets: word[start:stop]. The piece starts at start, and goes up to, but not including, stop.

The picture shows the first slice. Think of the numbers as being between the characters, at their left edges. word[2:5] cuts at 2 and at 5, and takes what is between. This is why word[:3] + word[3:] is always the whole word again: the two cuts meet at 3.

A slice never gives an IndexError. If the numbers are too big, Python simply takes what exists: "cat"[1:100] is at.

This is the same rule as range(): start included, stop not included.

String methods

A method is a function that belongs to a string. We write it after a dot: text.upper(). Here are the ones that you will use most.

Methods can be chained

Because each method gives a string, you can write another method after it. text.strip().title() first strips and then makes a title. Read it from left to right.

A very useful line when you read a name from the user: name = input("Name: ").strip().title(). It takes away the spaces that were typed by mistake, and it fixes the capital letters. So " aSHA rao " becomes "Asha Rao".

Common mistakes

• Expecting a method to change the variable. name.upper() on a line by itself does nothing that you can see. Write name = name.upper().
• Forgetting the brackets. name.upper without () is not an error, but it does not give the text. It shows something like <built-in method upper of str object>.
• Counting the stop position. In word[2:5] the character at position 5 is not in the slice.
• Mixing the two numbers. word[5:2] is empty, because the start is after the stop.
• Using title() on text with apostrophes. "it's".title() is "It'S". Be careful with such words.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Cut with slices. Cut out three pieces of a sentence. The checker confirms each one.
2. Build. Open the lab Clean the name. Turn a messy name into a clean one with the methods.
3. Stretch. Open the problem Change the first letter, and then write one line that reverses a word and makes it capital letters.

Remember these

• word[start:stop] takes the characters from start up to, but not including, stop.
• Leave out a number to go to the end: word[:3] and word[3:]. A step of -1 reverses.
• upper(), lower(), title(), strip() and replace() give a new string.
• Store the new string if you want to keep it.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What are "python"[1:4], "python"[:2] and "python"[-2:]?

2. How do you reverse "python"?

3. What does " hi ".strip().upper() give?

4. Does text.upper() change text?