Variables and Functions - Notebook

Variables

Variable assignment stores data in a named container. In Python, use = to assign values: x = 5. Variables are case-sensitive and can store numbers, strings, lists, and other datatypes. Variables in Python are mutable meaning the value can be modified.

The cells below set a variable x, print it, update it, and print it again.

x = 10

print(x)

10

x = 20

print(x)

20

Naming Variables

Variables names should be short and meaningful. Long names take time to type and can make code dense. Uninformative names can get confusing or accidentally overwritten.

Python rules for variable names include:

• Letters, numbers, and underscores (_) only
• Must start with a letter or underscore
• Snake case is preferred (e.g., my_var)
• No spaces, dashes, or dots
• Can not be reserved Python word (e.g., if, return)

See the table below for more examples.

Example	Valid?	Reason
age	✅ Yes	Starts with a letter, letters only
user_name	✅ Yes	Letters + underscore are allowed
temperature2	✅ Yes	Digits allowed, but not at start
_hidden	✅ Yes	Leading underscore allowed (often used for “private” values)
MAX_VALUE	✅ Yes	Uppercase is allowed, often used for constants
2nd_value	❌ No	Cannot start with a digit
user-name	❌ No	Dash (-) not allowed
file.path	❌ No	Dot (.) means “attribute,” not part of a name
my var	❌ No	Spaces not allowed
return	❌ No	return is a reserved Python keyword

Valid Variables

age = 10
print(age)

10

user_name = "John Smith"
print(user_name)

John Smith

temperature2 = 87
print(temperature2)

87

_hidden = "secret value"
print(temperature2)

87

MAX_VALUE = 1000000
print(MAX_VALUE)

1000000

Non-Valid Variables

2nd_value = 2
print(2nd_value)

  Cell In[10], line 1
    2nd_value = 2
    ^
SyntaxError: invalid decimal literal

user-name = "John Smith"
print(user-name)

  Cell In[11], line 1
    user-name = "John Smith"
    ^
SyntaxError: cannot assign to expression here. Maybe you meant '==' instead of '='?

file.path = "Users/dbugz/script.py"
print(file.path)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
Cell In[12], line 1
----> 1 file.path = "Users/dbugz/script.py"
      2 print(file.path)

NameError: name 'file' is not defined

my-var = 10
print(my-var)

  Cell In[13], line 1
    my-var = 10
    ^
SyntaxError: cannot assign to expression here. Maybe you meant '==' instead of '='?

return = "Reserved word"
print(return)

  Cell In[14], line 1
    return = "Reserved word"
           ^
SyntaxError: invalid syntax

Functions

Functions create reusable code which can simplify tasks, allow iterative runs, and can make your code much more powerful. Python comes with some builtin functions which will help demonstrate.

my_list = [1, 2, 27, 0]

print(my_list)

[1, 2, 27, 0]

type(my_list)

list

len(my_list)

4

min(my_list)

0

max(my_list)

27

str(my_list)

'[1, 2, 27, 0]'

int(my_list)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[22], line 1
----> 1 int(my_list)

TypeError: int() argument must be a string, a bytes-like object or a real number, not 'list'

lucky_number = "7020"

print(lucky_number)
type(lucky_number)

7020

str

lucky_int = int(lucky_number)

print(lucky_int)
type(lucky_int)

7020

int

lucky_float = float(lucky_number)

print(lucky_float)
type(lucky_float)

7020.0

float