第十二章 if Tests and Syntax Rules

最新推荐文章于 2025-09-13 10:32:19 发布

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本文深入探讨了Python中的if语句及其相关功能，包括条件判断、选择性执行块、默认值处理和复杂操作。通过实例展示了如何利用字典作为switch-case替代、get方法获取值以及错误处理策略，同时解释了Python语法、语句分隔符及表达式短路行为。文章还介绍了三元运算符的使用，并对比了if-else语句与三元表达式的实现方式。

1.if Statements

General Format

if test1: # if test
statements1 # Associated block
elif test2: # Optional elifs
statements2

else: # Optional else
statements3

>>> choice = 'ham'
>>> print({'spam': 1.25, # A dictionary-based 'switch'
... 'ham': 1.99, # Use has_key or get for default
... 'eggs': 0.99,
... 'bacon': 1.10}[choice])
1.99

Handling switch defaults：

>>> branch = {'spam': 1.25,
... 'ham': 1.99,
... 'eggs': 0.99}
>>> print(branch.get('spam', 'Bad choice'))
1.25
>>> print(branch.get('bacon', 'Bad choice'))
Bad choice

>>> choice = 'bacon'
>>> if choice in branch:
... print(branch[choice])
... else:
... print('Bad choice')
...
Bad choice

>>> try:
... print(branch[choice])
... except KeyError:
... print('Bad choice')
...
Bad choice

Handling larger actions：

def function(): ...
def default(): ...
branch = {'spam': lambda: ..., # A table of callable function objects
'ham': function,
'eggs': lambda: ...}
branch.get(choice, default)()

Python Syntax Revisited：

Statements execute one after another, until you say otherwise.
Block and statement boundaries are detected automatically
Compound statements = header + “:” + indented statements.
Blank lines, spaces, and comments are usually ignored.
Docstrings are ignored but are saved and displayed by tools.

Statement Delimiters: Lines and Continuations：

Statements may span multiple lines if you’re continuing an open syntactic pair. ()[]{}
Statements may span multiple lines if they end in a backslash. \
Special rules for string literals.triple-quoted

Truth Values and Boolean Tests:(使用文字，返回True False对象)

X and Y
X or Y
not X

>>> 2 or 3, 3 or 2 # Return left operand if true
(2, 3) # Else, return right operand (true or false)
>>> [] or 3
3
>>> [] or {}
{}


>>> 2 and 3, 3 and 2 # Return left operand if false
(3, 2) # Else, return right operand (true or false)
>>> [] and {}
[]
>>> 3 and []
[]

2. The if/else Ternary Expression

A = ((X and Y) or Z)
等价
A = Y if X else Z
等价
A = [Z, Y][bool(X)]  #not short-circuit