Course 4.1Lesson 4 of 5

Looking inside text

To look inside a text, find gives the position of the first match and minus one when there is none, rfind gives the last match, index does the same but raises a ValueError, and count says how many times. Tests such as isdigit, isalpha, isalnum, isspace, isupper and islower ask what kind of characters a text has. partition cuts a text at the first separator into three parts, which makes a line of the form key equals value easy to read, with a check for a missing separator or an empty key. Always check a line before you trust it, and turn the text of a number into a number only when it has digits.

about 30 min1 simulation2 practice questions

Lesson 4 of 5Show all lessons

Try the code and the simulations beside the lesson.

What you will be able to do

After this lesson you can

find where a piece is in a text, and how often it appears,
test what kind of characters a text has,
cut a line at its first separator with partition, and
read lines of the form key=value, and refuse the bad ones.

Finding a piece

You know in, which says whether a piece is in a text. Sometimes you need to know where.

Predict first. What will each line show? Then run it.

Method	What it gives
`text.find("cat")`	the position of the first match, or `-1` if there is none
`text.rfind("the")`	the position of the last match
`text.index("sat")`	like `find`, but a `ValueError` if it is missing
`text.count("at")`	how many times the piece appears

For "the cat sat on the mat": find("cat") is 4, find("dog") is -1, rfind("the") is 15 (the second the), and count("at") is 3 (cat, sat, mat). Positions start at 0, as always, and they count letters, spaces included.

As with the search of a list, -1 means "not found", and you must test for it before you use it as a position. A very common mistake is to use text[text.find("x")] when there is no x: the answer -1 quietly gives the last letter.

What kind of characters?

A family of methods answers yes or no about all the characters of a text.

Method	True when
`isdigit()`	every character is a digit (and there is at least one)
`isalpha()`	every character is a letter
`isalnum()`	every character is a letter or a digit
`isspace()`	every character is white space
`isupper()`, `islower()`	the letters are all capitals, or all small

The empty text is False for all of them. And a text with a minus sign or a decimal point, such as "-5" or "3.5", is not isdigit(), which surprises beginners. For numbers with a sign, use try and int() instead.

Cutting at a separator: partition

split cuts at every separator. Sometimes you want to cut only at the first one, into three parts: before, the separator, and after. That is partition.

"name=Asha".partition("=") gives ("name", "=", "Asha"), a tuple that you can unpack: key, sep, value = .... If the separator is missing, you get ("name", "", ""): the separator part is empty, which tells you.

The position of the = is also what find("=") gives, because the positions start at 0.

Reading a messy line

Here is a real job: read lines of the form key=value. Some lines are good. Some are broken.

At the end, the values that are made of digits become numbers, with isdigit() and int(). The value 9x is left as text, because it is not all digits.

Notice the care. A program that reads real files must expect broken lines, and keep going. The two checks (sep and key) are cheap, and prevent most of the trouble.

Common mistakes

Using find without testing for -1.
isdigit() on a number with a sign or a point. It is False.
Not cleaning the key. Name and name become two different keys.
Using split("=") when the value may contain an =. partition cuts only at the first one.
Forgetting the empty text ("".isdigit() is False).
Trusting a line before you check it.

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:

Warm-up. Open the lab Find and count. You use find, rfind and count, and the checker confirms the answers.
Build. Open the problem Read a line and the problem What kind of text?
Stretch. Open the lab Search the contact book from the projects course, and notice which methods of this lesson would help.

Remember these

find gives a position or -1. index raises an error. count says how many.
isdigit, isalpha, isalnum, isspace, isupper and islower test the kind of characters.
partition cuts at the first separator into three parts. An empty separator part means that it was missing.
Check a line before you trust it, and keep going when a line is bad.