Python Lambda 函数：新手指南

fn = lambda x, y: x + y
print(fn)

# <function <lambda> at 0xe508b8>技术支持

# Example: add two numbers using standard Python function
def add_numbers(x, y):
    return x + y技术支持

# Example: add two numbers using a lambda function
fn = lambda x, y: x + y
print(fn)

# <function <lambda> at 0xbfb968>技术支持

# Use filter with lambda function
numbers = [1, 2, 3, 4, 5, 6, 7, 8]
evens = filter(lambda x: x % 2 == 0, numbers)
print(list(evens))  # print the list of the filter object to see the result

# [2, 4, 6, 8]技术支持

# Use map with lambda function
fruits = ['apple', 'banana', 'cherry']
lengths = list(map(lambda x: len(x), fruits))
print(lengths)

# [5, 6, 6]技术支持

numbers = [1, 10, -1, 3, -10, 5]
sorted_stuff = sorted(numbers)
print(sorted_stuff)

# [-10, -1, 1, 3, 5, 10]技术支持

# Sort according to absolute value
sorted_numbers_absolute = sorted(numbers, key=lambda x: abs(x))
print(sorted_numbers_absolute)

# [1, -1, 3, 5, 10, -10]技术支持

# Sort a list of tuples by the second element
data = [(1, 3), (2, 1), (4, 2)]
sorted_data = sorted(data, key=lambda x: x[1])
print(sorted_data)

# [(2, 1), (4, 2), (1, 3)]技术支持

import time

numbers = list(range(1, 1000000))

# Standard function
def double_standard(x):
    return x * 2

start = time.time()
map(double_standard, numbers)
print(time.time() - start)

# Lambda function
double_lambda = map(lambda x: x * 2, numbers)

start = time.time()
list(map(lambda x: x * 2, numbers))
print(time.time() - start)

# 3.504753112792969e-05
# 2.384185791015625e-05技术支持

import random
import string

# Generate a dictionary with elements of the form 'XX': number.
NUMBER_ITEMS = 1000000
items = {
    ''.join(random.choices(string.ascii_uppercase, k=2)): random.randint(1, 100)
    for _ in range(NUMBER_ITEMS)
}

# Standard function (extra definition step)
def sort_standard(item):
    return item[1]

print('Standard')
start = time.time()
sorted_items_standard = sorted(items, key=sort_standard)
print(time.time() - start)
print(sorted_items_standard[:5])
print()

# Lambda function
print('Lambda')
start = time.time()
sorted_items_lambda = sorted(items, key=lambda x: x[1])
print(time.time() - start)
print(sorted_items_lambda[:5])
print()

# Standard
# 0.00011610984802246094
# ['OA', 'VA', 'XA', 'IA', 'BA']
# Lambda
# 0.00011014938354492188
# ['OA', 'VA', 'XA', 'IA', 'BA']技术支持

# Example: Use lambda function with built-in Python method reduce.
from functools import reduce

numbers = [5, -6, 2, 7]
total = reduce(lambda x, y: x + y, numbers)
print(f'The sum of the numbers is {total}.')

# The sum of the numbers is 8.技术支持

# Example: Use lambda function with built-in Python method zip.
list1 = [1, 2, 3]
list2 = [4, 5, 6]

# Using zip and a lambda function to multiply corresponding elements
result = list(map(lambda x: x[0] * x[1], zip(list1, list2)))

print(f'The result of multiplying corresponding elements is {result}.')

# The result of multiplying corresponding elements is [4, 10, 18].技术支持

# Sample data: list of dictionaries representing sales records
sales_data = [
	{'fruit': 'peaches', 'price': 1.41, 'quantity': 3},
	{'fruit': 'pears', 'price': 1.21, 'quantity': 2},
	{'fruit': 'mangoes', 'price': 0.56, 'quantity': 3},
]

# Using a lambda function to calculate total sales for each record
transformed_data = list(
	map(
		lambda entry: {**entry, 'total_sales': round(entry['price'] * entry['quantity'], 2)},
		sales_data
	)
)

# Print the transformed data
for record in transformed_data:
	print(record)

# {'fruit': 'peaches', 'price': 1.41, 'quantity': 3, 'total_sales': 4.23}
# {'fruit': 'pears', 'price': 1.21, 'quantity': 2, 'total_sales': 2.42}
# {'fruit': 'mangoes', 'price': 0.56, 'quantity': 3, 'total_sales': 1.68}技术支持

# Complex logic in a lambda
result = lambda x: (x ** 2 + x - 1) / (x + 1 if x != -1 else 1)
print(result(5))  # Hard to understand

# 4.833333333333333技术支持

def complex_logic(x):
    if x == -1:
        return x ** 2 + x - 1
    return (x ** 2 + x - 1) / (x + 1)
print(complex_logic(5))

# 4.833333333333333技术支持

# Forgetting the required arguments
numbers = [1, 2, 3, 4]
squared = map(lambda: x ** 2, numbers)  # <-- Where is the input argument? Error: lambda missing argument技术支持

# Dividing without handling zero
divide = lambda x, y: x / y

safe_divide = lambda x, y: x / y if y != 0 else "undefined"
print(safe_divide(5, 0))

# undefined技术支持

# Forgetting to convert to a list
numbers = [1, 2, 3]
squared = map(lambda x: x ** 2, numbers)
print(squared)  # <-- squared is the map, not the result

# <map object at 0x106d2b0>

print(list(squared))  # list(squared) gives the result

# [1, 4, 9]技术支持

numbers = [1, 2, 3, 4, 5]

# Lambda function with print to debug intermediate values
filtered_numbers = filter(lambda x: (print(f'Checking: {x} -> {x >= 3}'), x >= 3)[1], numbers)

# Converting filter object to list to force evaluation
print(list(filtered_numbers))

# Checking: 1 -> False
# Checking: 2 -> False
# Checking: 3 -> True
# Checking: 4 -> True
# Checking: 5 -> True
# [3, 4, 5]技术支持