Python 基本语法和缩进：完整的初学者指南-Python教程-PHP中文网

Python Basic Syntax and Indentation: The Complete Beginner

当你第一次学习编程时，Python 因一个特殊原因而脱颖而出：它的设计理念是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一个组织良好的文档。

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.")

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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")

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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

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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

我们来看看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

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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")

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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")

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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"

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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")

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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!")

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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

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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：

# 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.")

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