Variables, Data Types, and Coding Standards

Course: CSCI 1250

Assignment: Assignment 3 - Variables, Data Types, and Coding Standards

Variables

• Containers for storing data
• The value can change when a variable is mutable
• Because Python is dynamically typed, variables must be declared and assigned in the same statement
  • You can use None if you don’t want a value yet.
• In Python, you need a name and a value for a variable.
  • You can optionally add a type, such as str

cheese: str = "mozarella"
#       ^ optional type

Assignment

• Assignment operator (=)
  • The value on the right is assigned to the variable on the left.
  • IE: cheese = "mozarella"
• Variables can be re-assigned to any type.
  • If you have a type hint for the variable, your IDE should stop you, but Python itself doesn’t care.

Data Types

• Numeric data and non-numeric data
  • int → integers
  • float → decimal numbers
  • str → a list of characters; text data
  • bool → True or False

age = 69 # int
gpa = 0.1 # float
name = "skibidi toilet" # str
is_cool = True # bool

Type Checking

• You can check the type of a variable with the type() function in Python.
• You can use this in a conditional expression, such as:

x = 3.0
 
print(type(x)) # <class 'float'>
print(type(x) === float) # True

Type Conversion (Casting)

• Changes the data type of a variable to another
• Normally you’d have to use a casting operator, such as as
• In Python, you have to use the casting functions, such as int(), float(), and str()
  • These functions can raise errors when converting invalid values
  • float → int will truncate

x = "some string"
x = int(x) # ValueError: invalid ltieral for int()
 
y = 2.99
y = int(y) # ⌊2.99⌋ = 2

Math

Operators

• + → addition
• - → subtraction
• * → multiplication
• / → division
• // → floor division
• % → modulus
• ** → exponentiation

Division

• In Python, division between two integers always results in a float, unless you use integer/floor division //:

x = 15
y = 7
 
x / y # 2.14... (float)
x // y # 2 (int)

Modulo

• Returns the remainder of a division operation:

4 % 2 # 0
4 % 3 # 1

Strings

• Represented by the str class in Python
• Concatenation → combining two or more strings
  • You can only concatenate strings with other strings (or you raise a TypeError)
  • IE: "me" + "ow" → "meow"
• Interpolation → inserting different things into a string
  • You can use the .format method on strings, or an f-string
  • IE: f"i am {x} years old (where x is an int)

Methods

• You can get the character from a certain index using the str[idx] syntax.
  • IE: "Hello"[0] → "H", "Hello"[-1] → "o"
• You can get the length of a string using the len() function
  • IE: len("ABC") → 3
• You can slice strings using the str[start:stop:step] syntax.
  • The stop option is exclusive, and you can skip options
  • IE: "Hello"[0:4] → "Hell", "Hello"[0::4] → "Ho"

You can reverse a string with slicing by using [::-1]

Coding Standards

• The style guide for Python is known as PEP 8
  • Use # for inline (single-line) comments
  • Use """ for block (multi-line) comments
  • Use snake_case for naming variables/functions; PascalCase for naming classes
  • Empty lines between functions/classes; two empty lines after import statements
  • Keep lines $\approx 80$ characters

Comments should describe the why not the what