A first introduction to Python

1. Basics

Python is a newer programming language that is designed to be flexible and accessible. It is much slower than C, but modern computers are so fast that this doesn’t matter much for many of the things that you will do. Python is good at manipulating text, which is difficult in C; thus, Python is commonly used by scientists to do data analysis.

You can run Python code in two different ways:

• Type python at the command line, then type Python code directly into the terminal. The computer will execute each line as you input it. (This environment is called a “REPL”, for “Read, Evaluate, Print Loop”).

• Create a text file containing Python code. Conventionally, these files have names ending in .py, for instance program.py. You then run them by saying python program.py at the command line.

Here is a very simple but complete Python program that you can try:

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print ("Hello, world!")

2. Math and variables

Here’s a slightly more complicated program that introduces how to do math in Python:

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from numpy import *

a = float(input("Enter a number: "))
b = float(input("Enter another number: "))
c = sqrt(a**2 + b**2)

print ("The hypotenuse of a triangle with legs of length",a,"and",b,"is",c)

There are some new things here.

Line 1 imports functions from the numpy module. One of the great strengths of Python is the enormous range of modules available for it that do different things. NumPy, short for numerical Python, is a set of mathematics functions. We will need it in order to have access to the sqrt function which we’ll use later.

Line 3 sets the variable a equal to a number that the user enters. The function input() reads some text from the keyboard. So writing a = input("Enter a number: ") prompts the user to type something, and then stores whatever the user types in the variable a.

So, what’s this float business?

Variables and other objects in Python can have many different types. A variable like a can represent an integer, a decimal number (called float, for “floating-point”, i.e. scientific notation), some text (called a “string”), a list of numbers or strings of text, or even more complicated things.

The input() function returns a string – that is, text. Note that there is a difference between the number 5.0 and a string of text comprised of the characters “5.0”. Specifically, the computer knows how to do mathematics with numbers, but doesn’t know how to do math with text – even if that text describes a number. So, in order to do math with the things we have read from the keyboard, we must convert them into numbers. The float() function converts text into floating-point (i.e. decimal) numbers, which does exactly what we want here.

So line 3:

• Prompts the user to enter a number
• Converts whatever they type to a value of type float
• Stores that value in the variable a.

Important: The = operator in Python is called the assignment operator. It does not mean that one thing must forever be equal to another thing. Instead, it does something much more mundane: it takes the value on the right-hand side of the = and stores it in the variable on the left-hand side of the =. Thus, statements like x = x + 1 are perfectly valid Python: this just means “make x bigger by 1”.

Line 4 does the same thing for another number, storing it in b.

Line 5 does some mathematics. Notice that ** is the exponentiation operator, so a**2 means $a^2$, not a^2. Remember line 1, where we imported the numpy module? The sqrt function lives inside it, which we use now.

Finally, line 7 prints out the result. Notice that you can easily mix text and numbers in the print() function.

3. Loops: repeating things multiple times

Here’s another program that introduces looping structures: commands that tell the computer to repeat chunks of code. Clever use of these is the key to programming: the whole point of using a computer is that it can do repetitive, tiresome mathematical tasks quickly.

The for statement tells the computer to repeat a block of code multiple times. Python has a special type of object called an iterable, which you can think of as “an instruction for how to repeat something”.

For now, we need one iterable, which is the range function. It generates a range of numbers; range(5) generates the set of numbers (0, 1, 2, 3, 4).

See if you can guess what this program does before you run it:

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for i in range(20):
    print (i,"squared is",i*i)
print ("All done!")

Then run this code, and see how it works!

Notice a few things:

• There is a colon after the for statement. Statements like for, if, and while are called control statements. They tell the computer how to repeat, or whether to run, a block of code that follows them. In Python, they end with a colon.

• The block of code to be repeated is indented. Indentation in Python is used to describe blocks of code, and Python will complain if you don’t indent every line in a block of code by the same amount. Tabs and spaces are not the same: just because two things look like they’re indented the same amount, if one uses tabs and another uses spaces, Python will complain.

Here’s another way to do the same thing:

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i = 0
while (i < 20):
    print (i,"squared is",i*i)
    i = i + 1
print ("All done!")

You may end a loop prematurely using break.

4. Conditional statements

Here’s one more program that illustrates how to use the if statement to make the computer do one thing or another.

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# use the int() function to change text to integer
a = int(input("Enter an integer: "))  

if (a > 0):
    print ("The number is positive")
if (a < 0):
    print ("The number is negative")
if (a == 0):
    print ("The number is zero")

A couple of new things here. The biggest one is the if statement, of course. Note the use of == rather than = to check for equality. Single-equals is the assignment operator that changes the value of a variable; double-equals tests to see if two things are equal. Mixing these up is a common mistake.

Also notice the text beginning with #. This is a comment: text that is completely ignored by the compiler and does nothing, but is included just as a note to anyone reading the code. Use comments liberally; it’s essential that you or someone else reading your code can figure out what it does. At a minimum, any program you submit should have a comment at the top telling who wrote it and what it does.