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Python 基本语法和缩进：完整的初学者指南

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Python 基本语法和缩进：完整的初学者指南

当你第一次学习编程时，python 因一个特殊原因而脱颖而出：它的设计目的几乎像英语一样阅读。与使用大量符号和括号的其他编程语言不同，python 依赖于简单、干净的格式，使您的代码看起来像组织良好的文档。

将 python 的语法视为语言的语法规则。正如英语有关于如何构造句子以使含义清晰的规则一样，python 也有关于如何编写代码以便人类和计算机都能理解的规则。

理解python的基本语法

构建模块

让我们从最简单的 python 语法元素开始：

# this is a comment - python ignores anything after the '#' symbol
student_name = "alice"    # a variable holding text (string)
student_age = 15         # a variable holding a number (integer)

# using variables in a sentence (string formatting)
print(f"hello, my name is {student_name} and i'm {student_age} years old.")

在此示例中，我们使用了 python 的几个基本元素：

评论（以#开头的行）
变量（student_name 和 student_age）
字符串格式（f"..." 语法）
打印功能

基本操作

python可以像计算器一样进行计算和比较：

# basic math operations
total_score = 95 + 87    # addition
average = total_score / 2 # division

# comparisons
if student_age >= 15:
    print(f"{student_name} can take advanced classes")

python 的核心：理解缩进

这就是 python 真正独特的地方：python 使用缩进，而不是使用括号或特殊符号将代码组合在一起。乍一看这可能看起来很奇怪，但它使 python 代码异常清晰易读。

缩进如何创建结构

将缩进想象为组织详细大纲的方式：

def make_sandwich():
    print("1. get two slices of bread")  # first level
    if has_cheese:
        print("2. add cheese")           # second level
        print("3. add tomatoes")         # still second level
    else:
        print("2. add butter")           # second level in else block
    print("4. put the slices together")  # back to first level

每个缩进块都告诉 python“这些行属于一起”。这就像在大纲中创建一个子列表 - “if has_cheese:”下缩进的所有内容都是该条件的一部分。

缩进规则

让我们看看python缩进的关键规则：

def process_grade(score):
    # rule 1: use exactly 4 spaces for each indentation level
    if score >= 90:
        print("excellent!")
        if score == 100:
            print("perfect score!")

    # rule 2: aligned blocks work together
    elif score >= 80:
        print("good job!")
        print("keep it up!")  # this line is part of the elif block

    # rule 3: unindented lines end the block
    print("processing complete")  # this runs regardless of score

嵌套缩进：更深入

随着您的程序变得更加复杂，您通常需要多级缩进：

def check_weather(temperature, is_raining):
    # first level: inside function
    if temperature > 70:
        # second level: inside if
        if is_raining:
            # third level: nested condition
            print("it's warm but raining")
            print("take an umbrella")
        else:
            print("it's a warm, sunny day")
            print("perfect for outdoors")
    else:
        print("it's cool outside")
        print("take a jacket")

复杂的结构和压痕

让我们看一个更复杂的示例，它展示了缩进如何帮助组织代码：

def process_student_grades(students):
    for student in students:            # first level loop
        print(f"checking {student['name']}'s grades...")

        total = 0
        for grade in student['grades']: # second level loop
            if grade > 90:              # third level condition
                print("outstanding!")
            total += grade

        average = total / len(student['grades'])

        # back to first loop level
        if average >= 90:
            print("honor roll")
            if student['attendance'] > 95:  # another level
                print("perfect attendance award")

常见模式和最佳实践

处理多种条件

# good: clear and easy to follow
def check_eligibility(age, grade, attendance):
    if age < 18:
        return "too young"

    if grade < 70:
        return "grades too low"

    if attendance < 80:
        return "attendance too low"

    return "eligible"

# avoid: too many nested levels
def check_eligibility_nested(age, grade, attendance):
    if age >= 18:
        if grade >= 70:
            if attendance >= 80:
                return "eligible"
            else:
                return "attendance too low"
        else:
            return "grades too low"
    else:
        return "too young"

使用函数和类

class student:
    def __init__(self, name):
        self.name = name
        self.grades = []

    def add_grade(self, grade):
        # notice the consistent indentation in methods
        if isinstance(grade, (int, float)):
            if 0 <= grade <= 100:
                self.grades.append(grade)
                print(f"grade {grade} added")
            else:
                print("grade must be between 0 and 100")
        else:
            print("grade must be a number")

常见错误及其解决方法

缩进错误

# wrong - inconsistent indentation
if score > 90:
print("great job!")    # error: no indentation
    print("keep it up!")   # error: inconsistent indentation

# right - proper indentation
if score > 90:
    print("great job!")
    print("keep it up!")

混合制表符和空格

# wrong - mixed tabs and spaces (don't do this!)
def calculate_average(numbers):
    total = 0
    count = 0    # this line uses a tab
    for num in numbers:    # this line uses spaces
        total += num

练习：将它们放在一起

尝试编写这个程序来练习缩进和语法：

def grade_assignment(score, late_days):
    # Start with the base score
    final_score = score

    # Check if the assignment is late
    if late_days > 0:
        if late_days <= 5:
            # Deduct 2 points per late day
            final_score -= (late_days * 2)
        else:
            # Maximum lateness penalty
            final_score -= 10

    # Ensure score doesn't go below 0
    if final_score < 0:
        final_score = 0

    # Determine letter grade
    if final_score >= 90:
        return "A", final_score
    elif final_score >= 80:
        return "B", final_score
    elif final_score >= 70:
        return "C", final_score
    else:
        return "F", final_score

# Test the function
score = 95
late_days = 2
letter_grade, final_score = grade_assignment(score, late_days)
print(f"Original Score: {score}")
print(f"Late Days: {late_days}")
print(f"Final Score: {final_score}")
print(f"Letter Grade: {letter_grade}")