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Python is a typed language, data has a type, and different types of data cannot always be connected immediately and might need some conversion step before they can be used together. For instance if you add a number to a number, what should happen? If you add a number to a message, what do you expect will happen?

Agenda

In this tutorial, we will cover:

1. Types
2. Can mix integers and floats freely in operations.
3. Variables only change value when something is assigned to them.

Types

Every value in a program has a specific type.

Checking the Type

Use the built-in function type to find out what type a value has. This works on values as well as variables. But remember: the value has the type — the variable is just a label.

Check the type of values with the type() function:

print(type(52))
print(type(3.14159))

You can also check the types of variables

fitness = 'average'
print(type(fitness))

Methods

A value’s type determines what the program can do to it. Some operations may work

print(5 - 3)

And some operations may not work:

print('hello' - 'h')

For instance, you can use the + and * operators on strings.

full_name = 'Ahmed' + ' ' + 'Walsh'
print(full_name)
separator = '=' * 10
print(separator)

Some methods only accept specific types, or only work on specific types.

The built-in function len returns the length of your data. Which of the following would you expect to work? len(string)? len(int)?

print(len(full_name))
print(len(52))

Matching Types

Not all types support all operations, adding an integer to a string doesn’t make much sense:

print(1 + '2')

This does not work because it’s ambiguous: should 1 + '2' be 3 (a number) or '12' (a string)? Some types can be converted to other types by using the type name as a function.

print(1 + int('2'))
print(str(1) + '2')

Operation Support

Here is a quick chart showing which operations are allowed for each pair:

As you can see you can do 3 * "test" and "test" * 3, but it doesn’t work with floats.

Can mix integers and floats freely in operations.

Integers and floating-point numbers can be mixed in arithmetic. Python 3 (which we use) automatically converts integers to floats as needed.

print(f'half is {1 / 2.0}')
print(f'three squared is {3.0 ** 2}')

Variables only change value when something is assigned to them.

If we make one cell in a spreadsheet depend on another, and update the latter, the former updates automatically. However, this does not happen in programming languages.

variable_one = 1
variable_two = 5 * variable_one
variable_one = 2
print(f'first is {variable_one} and second is {variable_two}')

The computer reads the value of first when doing the multiplication, creates a new value, and assigns it to second. After that, second does not remember where it came from. Every computation happens line-by-line.

Question: Fractions

What type of value is 3.14159? How can you find out?

Solution

It is a floating-point number (often abbreviated “float”). It is possible to find out by using the built-in function type().

print(type(3.14159))
<class 'float'>

# Test out solutions here!

Question: Automatic Type Conversion

What type of value is the result of (3.25 + 4)?

Solution

It is a float: integers are automatically converted to floats as necessary.

result = 3.25 + 4
print(f'result is {type(result)}')

7.25 is <class 'float'>

# Test out solutions here!

Question: Choose a Type

What type of value (integer, floating point number, or character string) would you use to represent each of the following? Try to come up with more than one good answer for each problem. For example, in # 1, when would counting days with a floating point variable make more sense than using an integer?

1. Number of days since the start of the year.
2. Time elapsed from the start of the year until now in days.
3. Serial number of a piece of lab equipment.
4. A lab specimen’s age
5. Current population of a city.
6. Average population of a city over time.

Solution

The answers to the questions are:

1. Integer, since the number of days would lie between 1 and 365.
2. Floating point, since fractional days are required
3. Character string if serial number contains letters and numbers, otherwise integer if the serial number consists only of numerals
4. This will vary! How do you define a specimen’s age? whole days since collection (integer)? date and time (string)?
5. Choose floating point to represent population as large aggregates (eg millions), or integer to represent population in units of individuals.
6. Floating point number, since an average is likely to have a fractional part.

Question: Division Types

In Python 3, the // operator performs integer (whole-number) floor division, the / operator performs floating-point division, and the % (or modulo) operator calculates and returns the remainder from integer division:

print(f'5 // 3: {5 // 3}')
print(f'5 / 3: {5 / 3}')
print(f'5 % 3: {5 % 3}')

5 // 3: 1
5 / 3: 1.6666666666666667
5 % 3: 2

If num_subjects is the number of subjects taking part in a study, and num_per_survey is the number that can take part in a single survey, write an expression that calculates the number of surveys needed to reach everyone once.

Solution

We want the minimum number of surveys that reaches everyone once, which is the rounded up value of num_subjects/ num_per_survey. This is equivalent to performing a floor division with // and adding 1. Before the division we need to subtract 1 from the number of subjects to deal with the case where num_subjects is evenly divisible by num_per_survey.

num_subjects = 600
num_per_survey = 42
num_surveys = (num_subjects - 1) // num_per_survey + 1

print(num_subjects, 'subjects,', num_per_survey, 'per survey:', num_surveys)

600 subjects, 42 per survey: 15

# Test out solutions here!

Question: Strings to Numbers

Where reasonable, float() will convert a string to a floating point number, and int() will convert a floating point number to an integer:

print("string to float:", float("3.4"))
print("float to int:", int(3.4))

string to float: 3.4
float to int: 3

If the conversion doesn’t make sense, however, an error message will occur.

Input: Python

print("string to float:", float("Hello world!"))

Output

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: could not convert string to float: 'Hello world!'

Given this information, what do you expect the following program to do?

What does it actually do?

Why do you think it does that?

print("fractional string to int:", int("3.4"))

Solution

What do you expect this program to do? It would not be so unreasonable to expect the Python 3 int command to convert the string “3.4” to 3.4 and an additional type conversion to 3. After all, Python 3 performs a lot of other magic - isn’t that part of its charm?

int("3.4")

However, Python 3 throws an error.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '3.4'

Why? To be consistent, possibly. If you ask Python to perform two consecutive typecasts, you must convert it explicitly in code.

int(float("3.4"))

3

# Test out solutions here!

Question: Arithmetic with Different Types

Which of the following will return the floating point number 2.0? Note: there may be more than one right answer.

first = 1.0
second = "1"
third = "1.1"

1. first + float(second)
2. float(second) + float(third)
3. first + int(third)
4. first + int(float(third))
5. int(first) + int(float(third))
6. 2.0 * second

Solution

Answer: 1 and 4 give exactly 2.0. Answer 5 gives the value 2 which may be considered equivalent, but is not returning a float specifically.

# Test out solutions here!

Key points

• Every value has a type.

• Use the built-in function type to find the type of a value.

• Types control what operations can be done on values.

• Strings can be added and multiplied.

• Strings have a length (but numbers don’t).

• Must convert numbers to strings or vice versa when operating on them.

• Can mix integers and floats freely in operations.

• Variables only change value when something is assigned to them.

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Citing this Tutorial

1. The Carpentries, Helena Rasche, Donny Vrins, Bazante Sanders, 2022 Python - Basic Types & Type Conversion (Galaxy Training Materials). https://training.galaxyproject.org/training-material/topics/data-science/tutorials/python-types/tutorial.html Online; accessed TODAY
2. Batut et al., 2018 Community-Driven Data Analysis Training for Biology Cell Systems 10.1016/j.cels.2018.05.012

BibTeX

@misc{data-science-python-types,
author = "The Carpentries and Helena Rasche and Donny Vrins and Bazante Sanders",
title = "Python - Basic Types & Type Conversion (Galaxy Training Materials)",
year = "2022",
month = "09",
day = "28"
url = "\url{https://training.galaxyproject.org/training-material/topics/data-science/tutorials/python-types/tutorial.html}",
note = "[Online; accessed TODAY]"
}
@article{Batut_2018,
    doi = {10.1016/j.cels.2018.05.012},
    url = {https://doi.org/10.1016%2Fj.cels.2018.05.012},
    year = 2018,
    month = {jun},
    publisher = {Elsevier {BV}},
    volume = {6},
    number = {6},
    pages = {752--758.e1},
    author = {B{\'{e}}r{\'{e}}nice Batut and Saskia Hiltemann and Andrea Bagnacani and Dannon Baker and Vivek Bhardwaj and Clemens Blank and Anthony Bretaudeau and Loraine Brillet-Gu{\'{e}}guen and Martin {\v{C}}ech and John Chilton and Dave Clements and Olivia Doppelt-Azeroual and Anika Erxleben and Mallory Ann Freeberg and Simon Gladman and Youri Hoogstrate and Hans-Rudolf Hotz and Torsten Houwaart and Pratik Jagtap and Delphine Larivi{\`{e}}re and Gildas Le Corguill{\'{e}} and Thomas Manke and Fabien Mareuil and Fidel Ram{\'{\i}}rez and Devon Ryan and Florian Christoph Sigloch and Nicola Soranzo and Joachim Wolff and Pavankumar Videm and Markus Wolfien and Aisanjiang Wubuli and Dilmurat Yusuf and James Taylor and Rolf Backofen and Anton Nekrutenko and Björn Grüning},
    title = {Community-Driven Data Analysis Training for Biology},
    journal = {Cell Systems}
}
                   

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