Partial Application

Introduction

Partial application is a technique for fixing some arguments of a function, creating a new function with fewer parameters. The functools.partial module provides this capability.

Basic partial

from functools import partial

def multiply(a, b, c):
    return a * b * c

# Fix the first argument
double = partial(multiply, 2)
print(double(3, 4))  # 24

# Fix multiple arguments
always_five = partial(multiply, 2, 2, 1)
print(always_five())  # 4

Partial with Keywords

def power(base, exponent):
    return base ** exponent

square = partial(power, exponent=2)
cube = partial(power, exponent=3)

print(square(5))  # 25
print(cube(2))    # 8

Using partial in Data Processing

from functools import partial

def apply_tax(amount, tax_rate):
    return amount * (1 + tax_rate)

apply_10_percent = partial(apply_tax, tax_rate=0.10)
apply_20_percent = partial(apply_tax, tax_rate=0.20)

prices = [100, 200, 300]
with_tax = list(map(apply_10_percent, prices))
print(with_tax)  # [110, 220, 330]

Partial for Event Handlers

from functools import partial

def handle_click(button_id, event):
    print(f"Button {button_id} clicked at {event.x}, {event.y}")

button1_handler = partial(handle_click, "button1")
button2_handler = partial(handle_click, "button2")

# In event loop
button1_handler(mouse_event)

Functional Partial

from functools import partial

class Partial:
    def __init__(self, func, *args, **kwargs):
        self.func = func
        self.args = args
        self.kwargs = kwargs
    
    def __call__(self, *args, **kwargs):
        combined_args = self.args + args
        combined_kwargs = {**self.kwargs, **kwargs}
        return self.func(*combined_args, **combined_kwargs)
    
    def map(self, func):
        return Partial(lambda *a, **kw: func(self(*a, **kw)))

Practice Problems

1. Create partial function for string formatting
2. Use partial in map/filter operations
3. Create partial with keyword arguments
4. Implement custom partial class
5. Use partial for callback handlers