Operators, Functions and Methods

Overview

Questions

• What can I do with values of different types?
• What happens when I combine values of different types?
• What are functions and methods?

Objectives

• Use arithmetic operators on numbers and strings.
• Read and interpret error messages.
• Convert values from one type to another.
• Call built-in functions and string methods.

We continue with the variables from the previous episode:

PYTHON

name = "Lovelace"
age = 37
size = 1.7123

Operators

---

An object’s type determines what the program can do with it.

PYTHON

# Addition of integers
5 + 5

OUTPUT

10

PYTHON

# Multiplication
age * 100

OUTPUT

3700

The same operators also work on strings, but they do something different:

PYTHON

# Concatenation of strings
name + name

OUTPUT

'LovelaceLovelace'

PYTHON

# Concatenation of strings x times
name * 10

OUTPUT

'LovelaceLovelaceLovelaceLovelaceLovelaceLovelaceLovelaceLovelaceLovelaceLovelace'

We get an error if we try to add a number to a string.

PYTHON

# Does not work: the "+" operator cannot be applied
name + 10

ERROR

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

PYTHON

# Works: here, "10" is a string
name + "10"

OUTPUT

'Lovelace10'

Callout

Reading error messages

Error messages can look intimidating, but they are there to help you. Read the last line first: it tells you the type of the error (here a TypeError) and a short description of what went wrong. Above it, Python shows you the line of code where the error happened.

Dividing two numbers always results in a float:

PYTHON

# Division
size / age

OUTPUT

0.04627837837837838

PYTHON

type(size)

OUTPUT

float

Type conversion

---

Data that we read from files or from the web often arrives as text, even if it looks like a number.

PYTHON

number_of_articles = "50"

PYTHON

# Does not work: unsupported operand type(s) for /: 'str' and 'int'
number_of_articles / 10

ERROR

TypeError: unsupported operand type(s) for /: 'str' and 'int'

PYTHON

# First convert the string to an integer using int()
int(number_of_articles) / 10

OUTPUT

5.0

Similarly, str() converts a value to a string and float() converts it to a floating point number.

Functions and methods

---

The len() function tells us the length of an item. In the case of a string, for example, it tells us how many characters it contains.

PYTHON

len(name)

OUTPUT

8

Methods are special types of function that are always bound to an object, the thing on the left of the dot. For now, the difference between a function and a method is not that relevant.

PYTHON

name.upper()

OUTPUT

'LOVELACE'

PYTHON

# The string itself is not changed
name

OUTPUT

'Lovelace'

PYTHON

name.count("e")

OUTPUT

2

PYTHON

# Methods with two arguments
name.replace("lace", " is everywhere")

OUTPUT

'Love is everywhere'

Callout

Getting help

Use the built-in function help() to see the documentation of a function or method, e.g. help(len) or help(name.replace). In Jupyter, you can also put the cursor inside the parentheses and press Shift+Tab.

Challenge

Fix the error

A colleague wants to print how many articles a library has licensed, but the code below fails. What is wrong, and how can you fix it?

PYTHON

number_of_articles = 50
print("We licensed " + number_of_articles + " articles.")

Show me the solution

number_of_articles is an int, and Python cannot add an int to a str. Convert the number to a string first:

PYTHON

print("We licensed " + str(number_of_articles) + " articles.")

OUTPUT

We licensed 50 articles.

Challenge

String methods

Given title = "introduction to python", use string methods to

1. print the title in capital letters,
2. count how many times the letter "o" occurs,
3. replace "python" with "pandas".

Show me the solution

PYTHON

title = "introduction to python"
print(title.upper())
print(title.count("o"))
print(title.replace("python", "pandas"))

OUTPUT

INTRODUCTION TO PYTHON
4
introduction to pandas

Key Points

• The type of a value determines what an operator does with it.
• Read error messages from the bottom up: the last line names the error.
• Use int(), float() and str() to convert between types.
• Functions are called with function(arguments), methods with object.method(arguments).
• String methods return a new value and do not change the original string.