The Python Interpreter

Note

Learning objectives: Install the Python interpreter. Use the interpreter as a calculator. Explain what “returning” a value means.

Installing Python

  • Go to the Python Software Foundation web site and download the latest release for your operating system.

  • Different versions exist for Windows, Linux, MacOS, and others.

  • Your not very humble narrator even runs it on his cell phone.

    • (I need it for good reasons, like breaking into files I stole from your cell phone.)

  • You will see two flavors of Python, numbered Python 2 and Python 3.

    • They are very similar, but Python 3 is more advanced.

    • Think of Python 2 and Python 3 as separate programming languages.

Make it do math

Let’s start our Python interpreter and make it do stuff. Let’s make it do math.

>>> 7 + 5
12
>>> 12 - 5
7
>>> 12 - 5.0 # Now this is weird, right?  Where did that decimal point come from?
7.0
>>> 12 / 2
6
>>> 12 / 2.0
6.0
>>> 7 / 2 # I swear I am not making this up.  The computer isn't wrong.  You are.
3
_images/masha_and_tongue.png

What happens with 7 / 2.0 or 7.0 / 2 ?

Why does it do that?

Note

Computer programs read instructions from a disk into memory, then follow those instructions until they end or the computer runs out of resources.

  • You are running something called the Python interpreter. It’s a program for running other programs.

  • It takes Python instructions and runs them as soon as you press enter.

  • Whenever it “thinks” of an answer to something, it shows you right away. That’s called returning a value.

  • This is not how it will behave when you write a program and run it, but it is a great short-cut to figuring out how it works.

  • No matter what you give the interpreter, Python will spit back its simplified value.

>>> 5
5
>>> 5.0
5.0
>>> '5'
'5'

Notice how that last '5' is in quotes. I’ll be you thought it was a number. Here’s where the actual fun begins.

Arithmetic operators

Here are the math operators for adding, multiplying, getting remainders, etc.

Use this for reference.

Symbol

Operation

Result

Example

+

Add

Sum

2 + 5 = 7

-

Subtract

Difference

7 - 5 = 2

*

Multiply by

Product

2 * 4 = 8

/

Divide by

Quotient

8 / 4 = 2

**

To the power of

Power value

2 ** 3 = 8

%

Modulus

Remainder

14 % 6 = 2

Try some order-of-operations experiments. Go crazy. Use some parentheses.

In fact, use all the parentheses! No one is watching you.

>>> # What will this produce?
>>> 5 - 2 * (16 / 2 - 4) + (2 + 3)**2

>>> # What about this?
>>> 5 - (2 * (16 / 2 - 4) + (2 + 3)**2)

>>> # Unlike teenagers, computers always do what you say.
>>> # ...unless it's after a pound sign.
>>> # A pound sign tells Python to ignore everything after it.
>>> print('I will do this.') # ...but I will not do that.
I will do this.