Python Notes

Running a program

Sample program hello.py:

print("hello world")

Run program:

❯ python3 hello.py
hello world

Comments

Single line comment

#This is a comment
print("Hello, World!")

Inline comment

print("Hello, World!") #This is a comment

Multi-line comments

#This is a comment
#written in
#more than just one line
print("Hello, World!")

"""
This is a comment
written in
more than just one line
"""
print("Hello, World!")

Creating Variable

Python has no command for declaring a variable. A variable is created the moment you first assign a value to it.

x = 5
y = "John"
print(x)
print(y)

Dynamic Type

Variables do not need to be declared with any particular type, and can even change type after they have been set.

x = 4       # x is of type int
x = "Sally" # x is now of type str

Casting

x = str(3)    # x will be '3'
y = int(3)    # y will be 3
z = float(3)  # z will be 3.0

Getting Type of a Variable

You can get the data type of a variable with the type() function.

x = 5
y = "John"
print(type(x))
print(type(y))

Single Quote vs Double Quote

x = "John"
# is the same as
x = 'John'

Case sensitivity

Variable names are case-sensitive. This will create two variables:

a = 4
A = "Sally"
#A will not overwrite a

Variable naming convention

Variable name must follow the criteria:

A variable name must start with a letter or the underscore character
A variable name cannot start with a number
A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
Variable names are case-sensitive (age, Age and AGE are three different variables)

myvar = "John"
my_var = "John"
_my_var = "John"
myVar = "John"
MYVAR = "John"
myvar2 = "John"

Batch Assignment: Different values at different variable

Python allows you to assign values to multiple variables in one line:

x, y, z = "Orange", "Banana", "Cherry"
print(x)
print(y)
print(z)

The number of variable in left side must be equal to the number of values in right side. Otherwise, it will give error.

Batch Assignment: One value into multiple variables

x = y = z = "Orange"
print(x)
print(y)
print(z)

Unpacking a Collection

If you have a collection of values in a list, tuple etc. Python allows you extract the values into variables. This is called unpacking.

fruits = ["apple", "banana", "cherry"]
x, y, z = fruits
print(x)
print(y)
print(z)

Global Variable

Variables that are created outside of a function (as in all of the examples above) are known as global variables.

Global variables can be used by everyone, both inside of functions and outside.

Example:

x = "awesome"

def myfunc():
  x = "fantastic"
  print("Python is " + x)

myfunc()

print("Python is " + x)

Output:

Python is fantastic
Python is awesome

Global Variable using `global` Keyword

We can also expose a local variable from inside a function to act as a global variable using global keyword.

Example 1:

def myfunc():
  global x
  x = "fantastic"

myfunc()

print("Python is " + x)

Output:

Python is fantastic

Example 2:

x = "awesome"

def myfunc():
  global x
  x = "fantastic"

myfunc()

print("Python is " + x)

Output:

Python is fantastic

Built-in Data Types

| Category | Built-in Data Types | |

– |

| | Text Type | str | | Numeric Types | int, float, complex | | Sequence Types | list, tuple, range | | Mapping Type | dict | | Set Types | set, frozenset | | Boolean Type | bool | | Binary Types | bytes, bytearray, memoryview |

Get Type of a Variable

Print the data type of the variable x:

x = 5
print(type(x))

Output:

<class 'int'>

Setting Data-Type of a Variable

When we assign something into a variable, python dynamically set it’s data type according to the value we are trying to assign.

| Example | Data-Type | |

| | x = "Hello World" | str | | x = 20 | int | | x = 20.5 | float | | x = 1j | complex | | x = ["apple", "banana", "cherry"] | list | | x = ("apple", "banana", "cherry") | tuple | | x = range(6) | range | | x = {"name" : "John", "age" : 36} | dict | | x = {"apple", "banana", "cherry"} | set | | x = frozenset({"apple", "banana", "cherry"}) | frozenset | | x = True | bool | | x = b"Hello" | bytes | | x = bytearray(5) | bytearray | | x = memoryview(bytes(5)) | memoryview |

Setting Specific Data-Type

| Example | Data Type | |