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README.md

Basics Of Python

Motivation

Python is a high-level, interpreted, scripting language developed in the late 1980s by Guido van Rossum. Python is designed to be highly readable. Why choose python you may ask considering there are dozens of languages out there, here's why:

  • Python is popular

    Python has grown in popularity in the last decade. The 2019 stack overflow developer survey ranks Python as the 4th most popular language.

  • Python is interpreted

    Many languages are compiled, meaning the source code needs to be translated into a language your computer understands before it can be run. An interpreter skips this intermediate step and runs the program directly allowing a quicker development cycle. However, this can be a downside in some cases as interpreted languages tend to be slower.

  • Python is portable

    Since python is interpreted, code written for one platform will work on any other platform which has the interpreter installed.

  • Python is highly readable

    The code is simple, the syntax is easy to grasp and very readable. Additionally, it supports highly complex tasks which would otherwise be difficult to implement.

Basics

Python Interpreter

The python interpreter takes the code you write and converts it into a language that the computer understands. It does this each time you run a python script. There are 2 versions, python 2 & python 3. However, from January 1, 2020 python 2 will no longer be maintained. From now onwards, the term python refers to python 3.

Launch a terminal to get started

  • In Windows, it is called the Command Prompt.
  • In macOS or Linux, it is called the Terminal.

Open up the python interpreter by typing the following in the terminal

$ python3

You should see something like this

Python 3.7.4 (default, Oct  4 2019, 06:57:26) 
[GCC 9.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Make sure you have Python 3 installed, as you can see above we have Python 3.x. If you still have issues, you can use this online interpreter.

You have now launched python in an interactive REPL environment

  • Reads the command you enter
  • Evaluates and executes the command
  • Prints the output (if any) to the console
  • Loops back and repeats the process

Let's run our first command!

>>> print("Hello, World!")
Hello, World!

Comments

Write single-line comments with #

>>> # This is ignored by the python interpreter

Unfortunately, Python does not have a way to write multiline comments. However, you can still create multiline comments in 3 ways:

  1. >>> # This is a multiline comment
... # using the hash symbol
  2. >>> """
... This is a multiline comment
... using triple double quotes
... """
'\nThis is a multiline comment\nusing triple double quotes\n'
  3. >>> '''
... This is a multiline comment
... using triple single quotes
... '''
'\nThis is a multiline comment\nusing triple single quotes\n'

Ways 2 & 3 give you multiline functionality, but this isn't technically a comment. Be careful when you use these multiline comments, and when in doubt, just put the # on each subsequent line.

Data Types

Variables

Variables are nothing but reserved memory locations to store values. You can imagine them as a bucket to hold your information. You use the = operator to assign values to variables

>>> a = 10
>>> b = 20
>>> print(a)
10
>>> b
20

Python also allows you to assign values to multiple variables simultaneously

>>> a, b = 10, 20

Numbers

You have already seen a few numbers. Number data types store numeric values. Python supports 3 different numerical types

>>> a = -10		# int
>>> b = 3.14159		# float
>>> c = -2.2 + 5.3j	# complex

An integer is a whole number that can be positive, negative or zero.

A floating point number contains one fractional part after the decimal point.

A complex number is any number that can be written as a + bi, where i is the imaginary unit and a & b are real numbers.

You can check the type of a variable or value

>>> type(a)		# a = -10
<class 'int'>
>>> type(b)		# b = 3.14159
<class 'float'>
>>> type(c)		# c = -2.2 + 5.3j
<class 'complex'>

In python, there is no limit to how long an int value can be

>>> 12121212121212121212121212121212121212121212121212121212121212121212121 + 1
12121212121212121212121212121212121212121212121212121212121212121212122

Boolean

Boolean type may have one of two values, True or False.

>>> a = True	# bool
>>> b = False	# bool

The following elements are false:

  • False
  • None
  • 0 (whatever type i.e, int, float or complex)
  • Empty collections: "", (), [], {}
  • Objects from classes that have the special method __nonzero__
  • Objects from classes that implements __len__ to return False or 0

All values besides these are considered True in Python.

Strings

A string is a sequence of characters.

>>> str1 = "I like cats."	# str enclosed in double quotes
>>> str2 = 'I like dogs.'	# str enclosed in single quotes

When using quotes in strings, they need to be escaped using the \ operator

>>> str1 = 'I\'ll have what he is having.'
>>> str2 = "\"To be or not to be\" - William Shakespeare"

If you only need to use one type of quotes in your string, you can enclose the string in alternative quotes

>>> str1 = "I'll have what he is having."		# string with single quotes enclosed in double quotes
>>> str2 = '"To be or not be" - William Shakespeare'	# string with double quotes enclosed in single quotes

If you do not want characters prefaced by \ to be interpreted as special characters, you can use raw strings by adding an r

>>> print('C:\some\name')
C:\some
ame
>>> print(r'C:\some\name')
C:\some\name

Operators

Arithmetic Operators

Arithmetic operators are used with numeric values to perform common mathematical operations. The most basic of them are the unary operators

>>> a = 5
>>> b = 3
>>> +a		# unary addition
5
>>> -b		# unary subtraction
-3

Generally, you'll be using binary operators

>>> 2 + 2	# addition
4
>>> 8 - 5	# subtraction
3
>>> 3 * 5	# multiplication
15
>>> 8 / 2	# division
4.0

The numbers 4, 5 and 15 are of type int, i.e, integer. The number 4.0 has a fractional part and is of type float.

By default, the division / returns a float. To get an integer result (discard fractional part) called floor division, you can use the // operator

>>> 11 / 4	# floating point division
2.75
>>> 11 // 4	# floor division
2

If both operands of // are integers, only then the result is of type int. Otherwise, the result is of type float.

>>> 10 // 3		# int // int => int
3
>>> 10.0 // 3		# float // int => float
3.0
>>> 10 // 3.0		# int // float => float
3.0
>>> 10.0 // 3.0		# float // float => float
3.0

There are 2 more arithmetic operators

>>> 5**2	# exponentiation, 5 squared
25
>>> 10 % 3	# modulus, returns remainder of division
1

Comparison Operators

Comparison operators compare values on either sides, these are also called relational operators.

>>> 5 == 5	# (a == b) returns True if a is equal to b else False
True
>>> 5 != 5	# (a != b) returns True if a is not equal to b else False
False
>>> 2 < 3	# (a < b) returns True if a is strictly less than b else False
True
>>> 2 > 3	# (a > b) returns True if a is strictly greater than b else False
False
>>> 7 >= 7	# (a >= b) returns True if a is greater than or equal to b else False
True
>>> 7 <= 4	# (a <= b) returns True if a is less than or equal to b else False
False

Assignment Operators

An assignment operator assigns a value to its left operand based on the value of its right operand. The basic assignment operator is equal = which assigns the value of its right operand to its left operand. The other assignment operators are shorthand for standard operations.

>>> num  = 8	# a = b		num = 8
>>> num += 3	# a = a + b	num = 11
>>> num -= 5	# a = a - b	num = 6
>>> num *= 6	# a = a * b	num = 36
>>> num /= 5	# a = a / b	num = 7.2
>>> num//= 3	# a = a // b	num = 2.0
>>> num**= 3	# a = a ** b	num = 8.0
>>> num %= 5	# a = a % b	num = 3.0

Logical Operators

Logical operators are used to combine conditional statements.

>>> a = True
>>> b = False
>>> a and b	# returns True if both the operands are True
False
>>> a or b	# returns True if either of the operand is True
True
>>> not a	# returns True if operand is False
False

There are more types of operators, but for now these will suffice. You can read up about them here.

Control Flow

if Statement

if statements are used for decision making. It will run the body of code only if the condition is true. It has the following syntax

if <expression>:
	<statement>

Note the use of colon : after the <expression>.

If <expression> is true, then <statement> is executed. If <expression> is false, then <statement> is skipped over and not executed.

>>> raining = True
>>> if raining:		# the condition is True
...     print("Stay at home")
... 
Stay at home

Say you also want to execute a block of code if the condition is not true, here comes else to the rescue

>>> raining = False
>>> if raining:		# the condition is False
...     print("Stay at home")
... else:
...     print("Let's go outside")
... 
Let's go outside

If you have multiple separate conditions, use elif

>>> age = 21
>>> if age < 4:		# False
...     print("Join nursery")
... elif age < 18:	# False
...     print("Stay in school")
... elif age < 24:	# True!
...     print("Work hard in university")
... elif age < 60:	# skips
...     print("Get working")
... else:			# skips
...     print("Time to retire")
... 
Work hard in university

There can be 0 or more elif parts and the else part is optional.

The keyword elif is short for else if.

At most, one of the code blocks will be executed. If an else clause isn't included, and all the conditions are False, then none of the blocks will be executed.

Python also has the conditional expression (also known as a ternary operator) which has the following syntax

A if condition else B

Often you can use the conditional expression to make your if statement concise

>>> raining = True
>>> "Stay at home" if raining else "Let's go outside"
'Stay at home'

while Loop

Loops execute a block of code number of times as long as a condition is met. The most basic example is the while loop

>>> i = 0
>>> while i < 5:	# loop over numbers from 0 through 4
...     print(i)
...     i += 1
... 
0
1
2
3
4

If you want to exit your loop in case a certain condition is met, you can use break

>>> i = 0
>>> while i < 5:
...     if i == 3:	# breaks the loop when i = 3
...         break
...     print(i)
...     i += 1
... 
0
1
2

If you don't want to exit the loop but want to skip the code block execution in case a certain condition is met, you can use continue

>>> i = 0
>>> while i < 5:
...     if i == 3:	# skips the rest of the code block when i = 3
...			i += 1	# causes an infinite loop otherwise!
...         continue
...     print(i)
...     i += 1
... 
0
1
2
4

for Loop

The for loop iterates over the items of any sequence in the order they appear. Unlike the while loop which depends on a condition being met, for loop depends on the elements it has to iterate.

>>> for c in "python":	# loop over a sequence of elements
...     print(c)
... 
p
y
t
h
o
n

To loop over numbers, use the range function

>>> for i in range(5):	# loop over numbers from 0 through 4
...     print(i)
... 
0
1
2
3
4

The range function generates a sequence of numbers over time. The sytax of a range function is as follows

range([start,] end [,step])
  • start (optional) Starting point of the sequence, defaults to 0
  • stop (required) Endpoint of the sequence, non-inclusive
  • step (optional) Step size of the sequence, defaults to 1
>>> for i in range(1, 10, 2):	# loop over numbers from 0 through 9 picking every 2nd value
...     print(i)
... 
1
3
5
7
9

Summary

We covered: