What is Python?

Learning Objectives

• Different Python environments
• Python commands
• Math operators
• Python Strings

What is Python?

Python was created in the late 1980s, primarily as a teaching and scripting language. Python has since become an essential tool for many programmers, engineers, researchers, and data scientists across academia and industry. Accordingly, a large ecosystem of discipline-specific tools have been built on top of it. Part 2 of this course will introduce you to the Scientific Python Stack (SciPy) a collection of open source software for scientific computing in Python.

Python prompt

The traditional, pehaps still the default way to to start using Python, is to open up a command line on your computer. From here, opening up a Python session is usually as easy as typing python into the command prompt and hitting enter.

$ python

Here is a representative output of what you might see when you launch Python, namely a brief summary of which distribution and version of Python you are using.

Python 2.7.9 (default, Mar  1 2015, 12:57:24)
[GCC 4.9.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.

Your first Python command!

After running the python command in the command line the prompt changed to >>>. This means that for now we may only use commands in the Python language. You don't have to type in >>> - Python will do that for you.

If you want to exit the Python console at any point, just type exit() or use the shortcut Ctrl + Z for Windows and Ctrl + D for Mac/Linux. Then you won't see >>> any longer.

For now, we don't want to exit the Python console. We want to learn more about it. Let's start with something really simple. For example, try typing some math, like 2 + 3 and hit enter.

2 + 3

5

Nice! See how the answer popped out? Python knows math! You could try other commands like:

• 4 * 5
• 5 - 1
• 40 / 2

As you can see, Python is a great calculator. If you're wondering what else you can do...here is a quick overview of the common mathematical operators:

• + (plus)

  • Adds two objects
  • 3 + 5 gives 8.

• - (minus)

  • Gives the subtraction of one number from the other; if the first operand is absent it is assumed to be zero.
  • -5.2 gives a negative number and 50 - 24 gives 26.

• * (multiply)

  • Gives the multiplication of the two numbers or returns the string repeated that many times.
  • 2 * 3 gives 6.

• ** (power)

  • Returns x to the power of y
  • 3 ** 4 gives 81 (i.e. 3 * 3 * 3 * 3)

• / (divide)

  • Divide x by y
  • 13 / 3 gives 4.333333333333333

• // (divide and floor)

  • Divide x by y and round the answer down to the nearest whole number
  • 13 // 3 gives 4
  • -13 // 3 gives -5

• % (modulo)

  • Returns the remainder of the division
  • 13 % 3 gives 1. -25.5 % 2.25 gives 0.75.

Evaluation Order

If you had an expression such as 2 + 3 * 4, is the addition done first or the multiplication and does it matter? It certainly does matter, and Python will obediently follow the convention that the multiplication should be done first. Alternatively we can use round brackets () to override the default operator precedence, as well as making our code easier to follow.

Numbers

We've begun using numbers in python and it seems pretty straight forward. However, let's pause for a moment and observe that not all number are created equal. In fact, there are a number of types of numbers in Python. Let's verify this, before we move on:

type(2)

An examples of an integer is 2 which is just a whole number.

type(2.0)

In programming we call decimal numbers floating point numbers (or floats for short). Also what happened when we wrote type(2)? We used a function and we will return to what these are later in the course. You also might wonder why the need for multiple types of number in Python. In fact, it is very useful. If you wrote a program that counted people as they scanned their tickets at a concert, would you be better off using integers or floating point numbers?

Strings

How about your name? Type your first name in quotes like this:

"rick astley"

'rick astley'

You've now created your first string! It's a sequence of characters that can be processed by a computer. The string must always begin and end with the same character. This may be single (') or double (") quotes (there is no difference!) The quotes tell Python that what's inside of them is a string.

Strings can be strung together (or concatenated). Try this:

"Hi there " + "Rick"

'Hi there Rick'

You can also multiply strings with a number:

"Rick" * 3

'RickRickRick'

Nice, huh? To see your name in uppercase letters, simply type:

"Rick".upper()

'RICK'

You just used the upper method on your string! A method (like upper()) is a sequence of instructions that Python performs on a given object ("Rick"), once you call it. The notation x.method() reminds us that the method is attached or belongs to the data type x.

If you want to know the number of letters contained in your name, there is a function for that too!

len("Rick")

4

Wonder why sometimes you call functions with a . at the end of a string (like "Ola".upper()) and sometimes you first call a function and place the string in parentheses? Well, in some cases, functions belong to objects, like upper(), which can only be performed on strings. In this case, we call the function a method. Other times, functions don't belong to anything specific and can be used on different types of objects, just like len(). That's why we're giving "Rick" as a parameter to the len function.

Errors

Let's try something new. Can we get the length of a number the same way we could find out the length of our name? Type in len(304023) and hit enter:

len(304023)

Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
TypeError: object of type 'int' has no len()

We got our first error! It says that objects of type "int" (integers, whole numbers) have no length. So what can we do now? Maybe we can write our number as a string? Strings have a length, right?

len(str(304023))

6

It worked! We used the str function inside of the len function. str() converts everything to strings.

• The str function converts things into strings
• The int function converts things into integers

Important: we can convert numbers into text, but we can't necessarily convert text into numbers - what would int('hello') be anyway?