Python if Statement

Python3 Conditional Control

In Python programming,ifthe if statement is the most basic and important conditional control structure. It is used to determine whether to execute a specific block of code based on whether the condition is true or false.

Python'sifif statement is very intuitive and readable. It defines code blocks through indentation, which is a major feature that distinguishes Python from other languages.

Word Definition： ifMeans "if", and is the entry point for conditional judgment.

Basic Syntax and Parameters

ifThe if statement is an independent code block that needs to be used with a conditional expression.

Syntax Format

if 条件表达式:
    代码块

Syntax Description

  • Conditional expression: Any expression that returns a boolean value (non-zero, non-empty strings, non-empty containers, etc. are considered true).
  • Colon: A colon must be added after the conditional expression.
  • Indentation: The code block must be indented (usually 4 spaces). The indented code belongs to the if statement's sub-block.

Return Value/Effect

  • No return value: The if statement is a control flow statement and does not return any value.
  • Effect: When the condition is True, execute the indented code block; otherwise, skip it.

Examples

Let's thoroughly master theifusage of the if statement through a series of examples from simple to complex.

Example 1: Basic Usage - Comparing Number Sizes

Examples

# Basic if statement
age = 18

if age >= 18:
    print("Adult")

print("Program finished")

Expected output:

Adult

Code explanation:

  1. age >= 18is the conditional expression and returns True.
  2. Because the condition is true, executeprint("成年人")。
  3. print("程序结束")is not inside the if indented block, so it will execute regardless of the condition.

Example 2: Checking if a String is Empty

Examples

# Check string
name = ""

# Empty string is False
if name:
    print(f"Hello, {name}")
else:
    print("Please enter a name")

# Non-empty string
name = "Tom"
if name:
    print(f"Hello, {name}")

Expected output:

请输入名字
你好, Tom

Code explanation:

  • In Python, empty strings, empty lists, empty dictionaries, etc., are considered False in conditional checks.
  • Non-empty strings are considered True.

Example 3: Multiple Condition Checks

Examples

# Compound condition check
score = 85

# Using logical operators
if score >= 60 and score < 90:
    print("Passed")

# Check multiple conditions (simplified syntax)
if 60 <= score < 90:  # Python-specific chained comparison
    print("Score is between 60-90")

# Use in to check
colors = ["red", "green", "blue"]
if "red" in colors:
    print("Contains red")

Expected output:

及格了
成绩在 60-90 之间
包含红色

Code explanation:

  • Python supports chained comparisons, such as60 <= score < 90。
  • inThe in operator is used to determine whether an element is in a sequence.

Example 4: Single-line if Expression (Ternary Operator)

Examples

# Python ternary expression
age = 20

# Format: value1 if condition else value2
result = "Adult" if age >= 18 else "Minor"
print(result)  # Output: Adult

# More complex example
x = 10
y = 20
max_val = x if x > y else y
print(f"Larger value: {max_val}")  # Output: Larger value: 20

Expected output:

成年
较大值: 20

Python's ternary expression is more concise than traditional languages, with the formatvalue1 if condition else value2。

Python3 Conditional Control

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