Recitation 10 — Intro to Python

Skills: 8

Introduction to Python

We've learned the basics in Pyret—expressions, functions, and data. Today, we will transition to Python, which uses different notation but similar concepts.

Function Definitions and Types

Basic Function Translation

Pyret Version:

fun celsius-to-fahrenheit(temp :: Number) -> Number:
  doc: "convert Celsius temperature to Fahrenheit"
  (temp * 9/5) + 32
end

Python Version:

def celsius_to_fahrenheit(temp: float) -> float:
    """convert Celsius temperature to Fahrenheit"""
    return (temp * 9/5) + 32

Key Differences:

• def vs fun
• Underscores vs hyphens in names
• Colons for type annotations
• Explicit return statement required
• Triple quotes for documentation
• Indentation instead of end

What happens if you forget the return statement in Python?

Practice

Convert this Pyret function to Python:

fun calculate-tip(bill :: Number, percentage :: Number) -> Number:
    doc: "calculate tip amount for a restaurant bill"
    bill * (percentage / 100)
end

Conditionals in Python

If-Else Translation

Pyret Version:

fun letter-grade(score :: Number) -> String:
    doc: "convert numeric score to letter grade"
    if score >= 90:
        "A"
    else if score >= 80:
        "B"
    else if score >= 70:
        "C"
    else if score >= 60:
        "D"
    else:
        "F"
    end
end

Python Version:

def letter_grade(score: float) -> str:
    """convert numeric score to letter grade"""
    if score >= 90:
        return "A"
    elif score >= 80:
        return "B"
    elif score >= 70:
        return "C"
    elif score >= 60:
        return "D"
    else:
        return "F"

Key Differences:

• elif instead of else if
• Each branch needs its own return
• Indentation determines code blocks

Practice

Write a Python function for shipping cost:

• Orders under $25: $5 shipping
• Orders $25-$75: $3 shipping
• Orders over $75: free shipping

Python Lists vs Pyret Lists

Creating and Basic Operations

Pyret vs Python Comparison:

Pyret:

my-list = [list: "red", "green", "blue", "yellow"]
first-color = my-list.first
rest-colors = my-list.rest
list-size = my-list.length()

Python:

my_list = ["red", "green", "blue", "yellow"]
first_color = my_list[0]
rest_colors = my_list[1:]
list_size = len(my_list)

Point out indexing differences (0-based vs first/rest) and built-in len() function.

Filter and Map Operations

Pyret Version:

import lists as L

numbers = [list: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = L.filter(lam(n): num-modulo(n, 2) == 0 end, numbers)
squares = L.map(lam(n): n * n end, numbers)

Python Version:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = list(filter(lambda n: n % 2 == 0, numbers))
squares = list(map(lambda n: n * n, numbers))

Key Differences:

• Square brackets vs [list: ...]
• lambda instead of lam
• Must wrap filter and map with list() in Python

Testing in Python

Pyret where vs Python pytest

Pyret Testing:

fun double(n :: Number) -> Number:
    doc: "double the input number"
    n * 2
where:
    double(5) is 10
    double(0) is 0
    double(-3) is -6
end

Python Testing:

import pytest

def double(n: float) -> float:
    """double the input number"""
    return n * 2

def test_double():
    assert double(5) == 10
    assert double(0) == 0
    assert double(-3) == -6
    
# For floating-point comparisons:
def test_double_approx():
    assert double(3.3) == pytest.approx(6.6)

Need to run the file with pytest; running with python alone won't run tests.

Key Differences:

• Separate test functions instead of where blocks
• assert statements for testing

Practice

Write tests for your shipping cost function.

For Loops and List Processing

Pyret (for each):

fun sum-numbers(numbers :: List<Number>) -> Number:
    doc: "sum all numbers in a list"
    var total = 0
    for each(n from numbers):
        total := total + n
    end
    total
end

Python:

def sum_numbers(numbers: list) -> float:
    """sum all numbers in a list"""
    total = 0
    for n in numbers:
        total = total + n
    return total

Key Differences:

• No var keyword needed
• Regular assignment = instead of :=
• Simpler for loop syntax

Wrap-Up

• What are the main syntax differences between Pyret and Python?
• When might you choose one language over the other?