Python Refresher

Because we’ll be using Python throughout the course

Comments and Strings

# This is a comment

"This is a string"

'A' # This is also a string

# Python has no char type

"""This is a

multi-line string,

not a comment."""

Variables (and Objects)

• Variables begin to exist when assigned for the first time
• Every variable is a reference to an object (no primitives)

x = 374 # int object

y = 1.5 # float object

z = "Hi" # str object

• Every object has an id, a type, and a value
• Variables do not have types in Python
• Objects have methods that can be called

Data Types

Commonly used:

• int similar to Integer class in Java
• float similar to Double class in Java
• str similar to String class in Java

Data structures:

• list [1, 2, 3, 4, 5]
• tuple (38.437424, -78.867912)
• set {"Red", "Yellow", "Blue"}
• dict {"JMU Purple": "#450084", "JMU Gold": "#CBB677"}

f-Strings

name = "Elizabeth"�print(f"Hello {name}!")�# Hello Elizabeth!��a = 5�b = 10�print(f"Five plus ten is {a + b} and not {2 * (a + b)}.")�# Five plus ten is 15 and not 30.��print(f"{math.pi:.5f}")�# 3.14159

Control Flow

if mood == "Happy":� print("You know it!")

while grade == 'A':� print("Go Dukes!")� # infinite loop, btw

votes = [1, 3, 2, 1, 2]�for vote in votes:� print(vote)

Indentation is required (part of the syntax)

Functions

def add(x, y):� print(f"x is {x} and y is {y}")� return x + y

​

add(5, 6) # prints "x is 5 and y is 6" and returns 11

# Another way to call functions is with keyword arguments.�# Keyword arguments can arrive in any order.�add(y=6, x=5)

Type Hints

# Identifiers can optionally be annotated with type information.�def add(x: int, y: int) -> int:� print(f"x is {x} and y is {y}")� return x + y

age: int = 20

status: str # You don't need to initialize to specify the type.�if age < 18:� status = "Minor"�else:� status = "Adult"

Scope

global_variable = 'I am available everywhere'

def some_function():� print(global_variable)� local_variable = "only available within this function"� print(local_variable)

​

# the following code will raise an error, because�# 'local_variable' exists only inside some_function�print(local_variable)

Modules Example: hw1.py

# A Python source file defines a module with the same name.�# Other modules can be imported using the import statement.

import csv # built-in module from standard library�import go_dukes # searches for the file go_dukes.py

# Modules typically consist of function definitions.

# Executed only when this module is the main program.

if __name__ == "__main__":� print("Testing Testing 1...2...3...")

Docstrings Google Python Style Guide

If the first line of a module or function is a string,�then the string is interpreted as documentation.

def select(pk):� """Execute an SQL statement that selects one row from your table.�� Args:� pk (int): The id of the row to select.�� Returns:� tuple: The row for the specified pk, or None if not found.� """

Standard Library

The Python Standard Library

• See, for example: Built-in Functions

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Python Module Index

• On HW1, we’ll use pprint and sqlite3
• The csv module is also very common

VS Code Quick Start Guide