Python - Flow Control

  • How can my programs do different things based on data values?

  • Write conditional statements including if, elif, and else branches.

  • Correctly evaluate expressions containing and and or.

Time estimation: 40 minutes
Level: Introductory Introductory
Supporting Materials:
Last modification: Sep 28, 2022
License: Tutorial Content is licensed under Creative Commons Attribution 4.0 International License The GTN Framework is licensed under MIT
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“Flow Control” is how we describe when we change the flow of code’s execution, based on some conditions. Here we’ll learn how to take different actions depending on what data out program sees, or how to run code only if some condition is true.


This tutorial is significantly based on the Carpentries Programming with Python and Plotting and Programming in Python, which are licensed CC-BY 4.0.

Adaptations have been made to make this work better in a GTN/Galaxy environment.


In this tutorial, we will cover:

  1. Comparators
  2. Conditionals
  3. Multiple Branches
  4. Combining Tests
  5. Inverting Conditions
  6. Ranges
  7. Exercises


In Python we have the following comparators to do compare two values

  • >: greater than
  • <: less than
  • ==: equal to
  • !=: does not equal
  • >=: greater than or equal to
  • <=: less than or equal to

They’re all “binary” comparators, we can only compare two values at a time.

print(37 < 38)
print(38 < 38)
print(39 < 38)

These print out True or False, these are the two possible values of the boolean datatype in Python.

We can use <= to check if it’s less than or equal to:

print(19 <= 20)
print(20 <= 20)
print(21 <= 20)

And we can use == for comparing numbers in Python

print(11 == 11)
print(11 != 11)
print(22 != 33)

And now that we can compare numbers, we can start doing useful things with them!


We can ask Python to take different actions, depending on a condition, with an if statement:

num = 37
if num > 100:
    print('not greater')

The second line of this code uses the keyword if to tell Python that we want to make a choice. If the test that follows the if statement is true, the body of the if (i.e., the set of lines indented underneath it) is executed, and “greater” is printed. If the test is false, the body of the else is executed instead, and “not greater” is printed. Only one or the other is ever executed before continuing on with program execution to print “done”:

A flowchart diagram of the if-else construct that tests if variable num is greater than 100.

Conditional statements don’t have to include an else. If there isn’t one, Python simply does nothing if the test is false:

num = 53
print('before conditional...')
if num > 100:
    print(f'{num} is greater than 100')
print('...after conditional')
Question: If behaviour

Try changing the num value and see what happens for different values.

What happens if num is a:

  1. 202
  2. 3.145
  3. “test”
  4. 100.000001
  1. Condition is activated!
  2. Nothing, but not because it is a float! Because it’s less than 100
  3. Traceback, a TypeError, you cannot compare strings with integers
  4. Condition is activated!

Multiple Branches

But what if you want more branches? What if you need to handle more cases? elif to the rescue!

We can chain several tests together using elif, which is short for “else if”.

if todays_temperature > 30:
    print("Wear shorts! Remember your sunscreen")
elif todays_temperature > 20:
    print("It's nice weather finally! Gasp!")
elif todays_temperature < 10:
    print("Time to bundle up!")
    print("Dress normally")

if/elif/else cases follow these rules:

  • must start with an if
  • can have 0 or more elif conditions
  • can have 0 or 1 else condition (if no else condition is supplied, it’s equivalent to else: <nothing>)

Each of these three sections is a branch, the code pauses, and chooses to go down one of the branches based on the conditions.

The following Python code uses elif to print the sign of a number.

num = -3

if num > 0:
    print(f'{num} is positive')
elif num == 0:
    print(f'{num} is zero')
    print(f'{num} is negative')

NB: To test for equality we use a double equals sign == rather than a single equals sign = which is used to assign values.

Combining Tests

We can also combine tests using and and or. and is only true if both parts are true:

a = 1
b = -1
if (a > 0) and (b <= 0):
    print('both parts are true')
    print('at least one part is false')
Question: Predict what happens

Predict the outcomes of the following values of a and b above. Predicting what you think the code will do is a useful skill to practice

  1. a = 0; b = -1
  2. a = 0; b = 10
  3. a = 4; b = -22
  4. a = 99; b = 99
  1. at least one part is false
  2. at least one part is false
  3. both parts are true
  4. at least one part is false

while or is true if at least one part is true:

a = 1
b = -1
if (a < 0) or (b > 0):
    print('at least one test is true')

True and False are special words in Python called booleans, which represent truth values. A statement such as 1 < 0 returns the value False, while -1 < 0 returns the value True.

True and False booleans are not the only values in Python that are true and false. In fact, any value can be used in an if or elif. This is commonly used to check, for instance, if a string is empty or if some data is provided:

if '':
    print('empty string is true')
if 'word':
    print('word is true')

You can also use it to check if a list is empty or full:

if []:
    print('empty list is true')
if [1, 2, 3]:
    print('non-empty list is true')
# The last statement is equivalent to:
if len([1, 2, 3]) > 0:
    print('non-empty list is true')

Or you can check if a number is zero, or non-zero:

if 0:
    print('zero is true')
if 1:
    print('one is true')

Inverting Conditions

Sometimes it is useful to check whether some condition is not true. The Boolean operator not can do this explicitly. After reading and running the code below, write some if statements that use not to test the rule that you formulated in the previous question. not is a unary (not binary) operator: it only takes a single value

if not '':
    print('empty string is not true')
if not 'word':
    print('word is not true')
if not not True:
    print('not not True is true')


Python makes it super easy to check if a number is within a range.

quality_score = 32 # Try out different values!

if quality_score > 40:
    print("Your data is a bit sus")
elif 20 < quality_score <= 40:
    print("Hey that looks ok")
elif 4 < quality_score <= 20:
    print("Oh you did nanopore sequencing")
    print("It shouldn't be *that* bad. Try again.")

There are two important points here:

  • 20 < x < 40 is equivalent to 20 < x and x < 40, checking both sides of the condition, to make sure it’s greater than one value and smaller than another
  • Note that we checked in the second case 20 < x and then in the third we had to check x <= 20. If we had not had a <= on one side, what would have happened to 20? It would have gone straight to else!



ifs, elifs and elses get evaluated in blocks. Look at the following code and list the lines that are part of a single block.

1.  if x:
2.      # ..
3.  if y:
4.      # ..
5.  elif z:
6.      # ..
7.  if q:
8.      # ..
9.  else:
10.     # ..
11. elif t:
12.     # ..
13. else e:
14.     # ..

“Blocks” of if/elif/elses

  • must start with an if
  • can have 0 or more elif conditions
  • can have 0 or 1 else condition (if no else condition is supplied, it’s equivalent to else: <nothing>)

The above blocks are parsed together, you could not insert a print anywhere within the blocks, but between the blocks it would work.

  • 1-2, Just an if by itself. There’s no elif, or else, so that’s the end of that block
  • 3-6, if and elif get evaluated, there is no else, so that’s the end of that block
  • 7-10, if and else is fine
  • 11-14, error! This is missing an if case, it will fail with a syntaxerror.
# Test code here.
Question: How Many Paths?

Consider this code:

if 4 > 5:
elif 4 == 5:
elif 4 < 5:

Which of the following would be printed if you were to run this code? Why did you pick this answer?

  1. A
  2. B
  3. C
  4. B and C

C gets printed because the first two conditions, 4 > 5 and 4 == 5, are not true, but 4 < 5 is true.

# Test code here.
Question: Close Enough

Write some conditions that print True if the variable a is within 10 of the variable b and False otherwise. Compare your implementation with your partner’s: do you get the same answer for all possible pairs of numbers?

There is a [built-in function abs][abs-function] that returns the absolute value of a number:

a = 5
b = 5.1

if abs(a - b) <= 10:
print(abs(a - b) <= 10)

This works because the Booleans True and False have string representations which can be printed.

# Test code here.
Question: Pitfalls

A integer number between 0 and 100 will be provided to this function. Answer these two questions:

  • Will it always print something? If not, which value(s) fail?
  • Can you find any numbers the programmer explicitly wanted to handle, that aren’t handled as expected?
num = 42 # Randomly chosen so the code will execute, try changing it around.
if num > 90:
    print("great score")
elif num < 32:
    print("Very cold")
elif num >= 86:
elif num == 86:
    print("It's exactly this value!")
elif 32 < num < 58:
    print("Getting warmer")
elif 57 < num <= 86:
    print("Everything else goes here")
  1. No, it won’t. 32 is the only value there that doesn’t print anything. You can either do x < 57 and later 57 <= x to test the bigger and smaller values, or you can make use x < 57 and 56 < x, which have the same results, but only with integers. If your code accepted a float, e.g. 56.5, both of those tests would be true. So x < 57 and later 57 <= x is the preferred way to write that.
  2. 86 is the most obvious solution to this, the programmer added a check specifically to see if the value was 86, but instead it’s caught by the previous case.
num = 42 # Randomly chosen so the code will execute, try changing it around.
if num > 90:
    print("great score")
elif num < 32:
    print("Very cold")
elif num >= 86:
elif num == 86:
    print("It's exactly this value!")
elif 32 < num < 58:
    print("Getting warmer")
elif 57 < num <= 86:
    print("Everything else goes here")

Complicated if/elif/else cases are common in code, you need to be able to spot these sort of issues. For example there are large if/else cases in the Galaxy codebase, sometimes nested even, and being ale to predict their behaviour is really important to being able to work with the code. Missing else cases are sometimes important, sometimes a bug, sometimes just the code hasn’t been implemented yet, which is why we always write good code comments!

Key points
  • Use if condition to start a conditional statement, elif condition to provide additional tests, and else to provide a default.

  • The bodies of the branches of conditional statements must be indented.

  • Use == to test for equality.

  • X and Y is only true if both X and Y are true.

  • X or Y is true if either X or Y, or both, are true.

  • Zero, the empty string, and the empty list are considered false; all other numbers, strings, and lists are considered true.

  • True and False represent truth values.

  • not can be used to invert the condition

Frequently Asked Questions

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Citing this Tutorial

  1. The Carpentries, Helena Rasche, Donny Vrins, Bazante Sanders, Python - Flow Control (Galaxy Training Materials). Online; accessed TODAY
  2. Batut et al., 2018 Community-Driven Data Analysis Training for Biology Cell Systems 10.1016/j.cels.2018.05.012

author = "The Carpentries and Helena Rasche and Donny Vrins and Bazante Sanders",
title = "Python - Flow Control (Galaxy Training Materials)",
year = "",
month = "",
day = ""
url = "\url{}",
note = "[Online; accessed TODAY]"
    doi = {10.1016/j.cels.2018.05.012},
    url = {},
    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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