Python Programming Basics

This tutorial is an introduction to Python basics such as how to assign a variable value and change that value. There is also a list of reserved keywords although use of these keywords is explained later.

Python Variable Names

Valid variable names are those that start with a letter (upper or lower case) or underscore. Valid variable names include:


Invalid variable names include those that start with a number, have a space in the name, or contain special characters such as:

 my Variable

Python Reserved Keywords

There are reserved keywords in Python that control the flow of the program and cause unique behavior. Built-in constants include:

 True     # True logical condition
 False    # False logical condition
 None     # Absense of a value

Note that True, False, and None are capitalized. Common reserved keywords include:

 and      # logical 'and' (both conditions True)
 as       # aliasing (e.g. with open(file) as f:)
 assert   # internal self-check (e.g. assert x=y)
 break    # break out of a loop
 class    # define a new class (variables and functions)
 continue # advance to next 'for' or 'while' loop
 def      # define a function
 del      # set a variable to 'None'
 elif     # 'else if' condition of 'if' statement 
 else     # 'else' condition of 'if' statement
 except   # catch errors
 finally  # 'try' exit code
 for      # 'for' loop
 from     # specify package element for loading
 global   # create a global variable
 if       # 'if' statement
 import   # load package
 in       # membership operator (e.g. x in y)
 is       # test for equal identification numbers
 lambda   # create anonymous function
 nonlocal # expand variable scope outside function
 not      # logical 'not'
 or       # logical 'or'
 pass     # null operator when statement is required
 print    # create display output line
 raise    # error notification (e.g. raise Exception('Error'))
 return   # return from function
 try      # 'try'...'except' code structure for error catching
 while    # loop until condition is not True
 with     # flow control for automatic clean-up
 yield    # intermediate return from function

There are also many built-in functions that should also be avoided when creating variable names. Some common functions are listed below.

 float    # convert number to floating point type
 id       # identification number
 int      # convert number to integer type
 len      # number of elements in a collection
 max      # maximum value
 min      # minimum value
 pow      # power (e.g. pow(x,2) or use x**2)
 range    # sequence generator
 round    # nearest integer value
 str      # convert to string type
 type     # object type

These reserved keywords should be avoided when creating variable names. Most Integrated Development Environments (IDEs) for Python and text editors for Python scripts will automatically highlight the reserved keywords such as the example below where reserved keywords are highlighted in orange and green.

from random import *
answer = randint(0,10)
correct = False
while not(correct):
    guess = int(input("Guess a number (0-10): "))
    if guess < answer:
        print("Too low")
    elif guess > answer:
        print("Too high")
        correct = True
 Guess a number (0-10): 5
 Too high
 Guess a number (0-10): 3
 Too high
 Guess a number (0-10): 1
 Too low
 Guess a number (0-10): 2

Python Objects

Python is an object-oriented programming language. Each Python object has defining properties including the Identity (a memory address), Type (e.g. string, integer, floating point number), and Value. Below is an example of assigning 'x' as an Integer and printing the object properties.

x=2            # create variable 'x'
print(x)       # print value     => 2
print(id(x))   # print id number => 1737348944
print(type(x)) # print type      => <class 'int'>

Python Data Types

Data can be stored a variety of formats. Fundamental Data Types include Numbers (e.g. int and float), Sequences (e.g. str, list, and tuple), and Mappings (dict). Below are a few examples:

x=2             # integer
y=2.02          # float
z='3'           # string
print(str(x)+z) # 23
print(x+int(z)) # 5

a=[x,y,z]       # list
print(a[1])     # 2.02

b=(x,y)         # tuple
print(b[1])     # 2.02

c={'m':7,'n':8} # dictionary
print(c['m'])   # 7

Python Math Expressions

Python has addition (+), subtraction (-), multiplication (*), division (/), and power (**) as standard mathematical operators. The Math module is available by importing the math package. The math module provides a number of common mathematical operators such as shown in the example below.

import math
print(math.sin(x)) # 9.265e-05

The Numpy module also has many of the mathematical operators. This course uses many NumPy and SciPy built-in values and functions for scientific computing. Below is an example of using a built-in value and a trigonometric function (sine). In this case, the NumPy package is renamed as np to shorten the future references (numpy.pi => np.pi).

import numpy as np
print(math.sin(x))  # 1.225e-16

The result of mathematical expressions may be slightly off the expected value such as the example above that should equal to zero for sin(pi). This is due to finite machine precision (32-bit or 64-bit) calculations that have some round-off error at the last stored digit.

Python Strings

Strings are a list of characters. Strings are combined with the addition (+) symbol and either the single or double quotes are used.

first = 'Tom'
last = "Smith"
name = first + ' ' + last
print(name)  # Tom Smith

Comments can are added to Python scripts with the hash (#) symbol. When included inside a string, the comment character becomes part of the text and is no longer a comment.

hashtag = '#learnpython'
print(hashtag) # prints: #learnpython

There are also special escape sequences such as newline (\n), tab indent (\t), double quote (\"), single quote (\'), and backslash (\\).

flight = 'Landing at\n O\'Hare International Airport'
# prints:
# Landing at
#  O'Hare International Airport

To avoid using special escape sequences such as newline, a raw string identifier (r) can be used before the string definition.

flight = r'Landing at\n O\'Hare International Airport'
# prints:
# Landing at\n O\'Hare International Airport

An easy way to convert a number to a string is with the built-in function str or with formatted output. However, this method may produce a number that has too many decimal places. Numbers can be customized in floating point (f), scientific notation (e), or percentage (%) form with format such as:

# import the number pi
import numpy as np
x = np.pi

# format pi
print(str(x))              # 3.141592653589793
print('{:.2f}'.format(x))  # 3.14
print('{:.10f}'.format(x)) # 3.1415926536
print('{:.5e}'.format(x))  # 3.14159e+00
print('{:.3%}'.format(x))  # 314.159%