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This module explains Python decision logic in general and the if statement in particular.

Table of contents


This module is one in a collection of modules on Python designed for teaching ITSE 1359 Introduction to Scripting Languages: Python at Austin Community College in Austin, TX.

What you have learned

You were introduced to decision logic in the earlier module titled Itse 1359-1210-The While Loop . In that module I told you that Python supports a stand-alone decision capability, the if statement, which would be discussed in a future module. This is that future module.

You learned about the relational and logical operators that are used in decision logic in the module titled Itse1359-1220-Operators .

What you will learn

In this module, you will learn about

  • the if statement
  • the if...else statement
  • nested if statements

Viewing tip

I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the Figuresand the Listings while you are reading about them.

(Note to blind and visually impaired students: Most of the Figures and all of the Listings inthis module are presented in plain text format and should be accessible using an audio screen reader or a braille display. Note however that the requiredindentation may not be properly represented by an audio screen reader.)


  • Figure 1 . Pseudo-code for an if statement.
  • Figure 2 . Output from the code in Listing 1.
  • Figure 3 . Visualization of the code in Listing 1.
  • Figure 4 . Pseudo-code for a Python if...else statement.
  • Figure 5 . Output from the code in Listing 2.
  • Figure 6 . Output from the code in Listing 3.


  • Listing 1 . Example of if statement usage.
  • Listing 2 . Example of if...else statement usage.
  • Listing 3 . Example of nested if statements.

Discussion and sample code

The if statement

The pseudo-code for a Python if statement is shown in Figure 1 .

Figure 1 . Pseudo-code for an if statement.
if expression is True: Execute one or more statements at same indentation levelExecute next statement

The expression must be one that will evaluate to either True or False.

If the expression evaluates to True, the indented statement(s) that follow the colon will be executed in sequence.

If the expression evaluates to False, those indented statements will be skipped and the next statement following the if statement will be executed.

Listing 1 shows an example of the use of an if statement.

Listing 1 . Example of if statement usage.
# Illustrates the if statement ##------------------------------- dogs = ["Affenpinscher","Afgan Hound","Akita"]if len(dogs)>2: print(dogs[0]) print(dogs[1]) print(dogs[2]) print("Length of dogs list = " + str(len(dogs)))if len(dogs) == 2: print(dogs[0]) print(dogs[1]) print(dogs[2])print(dogs)

Questions & Answers

do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
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what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
how did you get the value of 2000N.What calculations are needed to arrive at it
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Source:  OpenStax, Itse 1359 introduction to scripting languages: python. OpenStax CNX. Jan 22, 2016 Download for free at https://legacy.cnx.org/content/col11713/1.32
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