Python 中的表达式和语句有什么区别？

What is the difference between an expression and a statement in Python?

提问by wassimans

In Python, what is the difference between expressions and statements?

在 Python 中，表达式和语句有什么区别？

采纳答案by Sven Marnach

Expressionsonly contain identifiers, literalsand operators, where operators include arithmetic and boolean operators, the function call operator()the subscription operator[]and similar, and can be reduced to some kind of "value", which can be any Python object. Examples:

表达式只包含标识符、文字和运算符，其中运算符包括算术和布尔运算符、函数调用运算符()、订阅运算符[]等，并且可以简化为某种“值”，可以是任何 Python 对象。例子：

3 + 5
map(lambda x: x*x, range(10))
[a.x for a in some_iterable]
yield 7

Statements (see 1, 2), on the other hand, are everything that can make up a line (or several lines) of Python code. Note that expressions are statements as well. Examples:

声明（见 1、2) 另一方面，是可以组成一行（或几行）Python 代码的所有内容。请注意，表达式也是语句。例子：

# all the above expressions
print 42
if x: do_y()
return
a = 7

回答by Flavius

An expression is something that can be reduced to a value, for example "1+3"or "foo = 1+3".

表达式是可以简化为一个值的东西，例如"1+3"或"foo = 1+3"。

It's easy to check:

很容易检查：

print foo = 1+3

If it doesn't work, it's a statement, if it does, it's an expression.

如果它不起作用，它是一个语句，如果它起作用，它是一个表达式。

Another statement could be:

另一种说法可能是：

class Foo(Bar): pass

as it cannot be reduced to a value.

因为它不能减少到一个值。

回答by user225312

Though this isn't related to Python:

虽然这与 Python 无关：

An expressionevaluates to a value. A statementdoes something.

Anexpression评估为一个值。Astatement做某事。

>>> x + 2         # an expression
>>> x = 1         # a statement 
>>> y = x + 1     # a statement
>>> print y       # a statement (in 2.x)
2

回答by Walter Nissen

Python calls expressions "expression statements", so the question is perhaps not fully formed.

Python 将表达式称为“表达式语句”，所以这个问题可能还没有完全形成。

A statement consists of pretty much anything you can do in Python: calculating a value, assigning a value, deleting a variable, printing a value, returning from a function, raising an exception, etc. The full list is here: http://docs.python.org/reference/simple_stmts.html#

语句几乎包含您在 Python 中可以执行的任何操作：计算值、赋值、删除变量、打印值、从函数返回、引发异常等。完整列表在这里：http:// docs.python.org/reference/simple_stmts.html#

An expression statement is limited to calling functions (e.g., math.cos(theta)"), operators ( e.g., "2+3"), etc. to produce a value.

表达式语句仅限于调用函数（例如，math.cos(theta)"）、运算符（例如，“2+3”）等来生成值。

回答by dawg

Expression-- from the New Oxford American Dictionary:

表达——来自新牛津美式词典：

expression: Mathematicsa collection of symbols that jointly express a quantity : the expression for the circumference of a circle is 2πr.

表达式：数学中共同表达一个量的符号集合：圆周长的表达式为 2πr。

In gross general terms: Expressions produce at least one value.

总而言之：表达式至少产生一个值。

In Python, expressions are covered extensively in the Python Language ReferenceIn general, expressions in Python are composed of a syntactically legal combination of Atoms, Primariesand Operators.

在 Python 中，表达式在Python 语言参考中有广泛的介绍。一般来说，Python 中的表达式由Atoms、Primaries和Operators的语法合法组合组成。

Python expressions from Wikipedia

来自维基百科的 Python 表达式

Examples of expressions:

表达式示例：

Literalsand syntactically correct combinations with Operatorsand built-in functionsor the call of a user-written functions:

与运算符和内置函数或用户编写的函数调用的文字和语法正确的组合：

>>> 23
23
>>> 23l
23L
>>> range(4)
[0, 1, 2, 3] 
>>> 2L*bin(2)
'0b100b10'
>>> def func(a):      # Statement, just part of the example...
...    return a*a     # Statement...
... 
>>> func(3)*4
36    
>>> func(5) is func(a=5)
True

---

---

Statementfrom Wikipedia:

来自维基百科的声明：

In computer programming a statement can be thought of as the smallest standalone element of an imperative programming language. A program is formed by a sequence of one or more statements. A statement will have internal components (e.g., expressions).

在计算机编程中，语句可以被认为是命令式编程语言中最小的独立元素。程序由一个或多个语句的序列构成。语句将具有内部组件（例如，表达式）。

Python statements from Wikipedia

来自维基百科的 Python 语句

In gross general terms: Statements Do Somethingand are often composed of expressions (or other statements)

概括地说：语句做某事，通常由表达式（或其他语句）组成

The Python Language Reference covers Simple Statementsand Compound Statementsextensively.

Python 语言参考广泛涵盖了简单语句和复合语句。

The distinction of "Statements do something" and "expressions produce a value" distinction can become blurry however:

然而，“语句做某事”和“表达式产生值”的区别可能变得模糊：

• List Comprehensionsare considered "Expressions" but they have looping constructs and therfore also Do Something.
• The ifis usually a statement, such as if x<0: x=0but you can also have a conditional expressionlike x=0 if x<0 else 1that are expressions. In other languages, like C, this form is called an operator like this x=x<0?0:1;
• You can write you own Expressions by writing a function. def func(a): return a*ais an expression when used but made up of statements when defined.
• An expression that returns Noneis a procedure in Python: def proc(): passSyntactically, you can use proc()as an expression, but that is probably a bug...
• Python is a bit more strict than say C is on the differences between an Expression and Statement. In C, any expression is a legal statement. You can have func(x=2);Is that an Expression or Statement? (Answer: Expression used as a Statement with a side-effect.) The assignment statement of x=2inside of the function call of func(x=2)in Python sets the named argument ato 2 only in the call to funcand is more limited than the C example.

• 列表推导式被认为是“表达式”，但它们具有循环结构，因此也可以做一些事情。
• Theif通常是一个语句，例如if x<0: x=0但您也可以使用条件表达式，例如x=0 if x<0 else 1表达式。在其他语言中，比如 C，这种形式被称为这样的操作符x=x<0?0:1;
• 您可以通过编写函数来编写自己的表达式。def func(a): return a*a是使用时的表达式，但在定义时由语句组成。
• 返回的表达式None是 Python 中的一个过程：从def proc(): pass语法上讲，您可以将其proc()用作表达式，但这可能是一个错误...
• Python 在表达式和语句之间的差异上比 C 更严格一些。在 C 中，任何表达式都是合法的语句。你可以有func(x=2);那是表达式还是语句？（答案：表达式用作具有副作用的语句。）Pythonx=2中的函数调用内部的赋值语句仅在调用func(x=2)中将命名参数设置a为 2，func并且比 C 示例更受限制。

回答by abifromkerala

A statement contains a keyword.

语句包含关键字。

An expression does not contain a keyword.

表达式不包含关键字。

print "hello"is statement, because printis a keyword.

print "hello"is 语句，因为printis 是一个关键字。

"hello"is an expression, but list compression is against this.

"hello"是一个表达式，但列表压缩反对这个。

The following is an expression statement, and it is true without list comprehension:

以下是一个表达式语句，在没有列表推导式的情况下为真：

(x*2 for x in range(10))

回答by Emmanuel Osimosu

Statements represent an action or command e.g print statements, assignment statements.

语句表示一个动作或命令，例如打印语句、赋值语句。

print 'hello', x = 1

Expression is a combination of variables, operations and values that yields a result value.

表达式是产生结果值的变量、操作和值的组合。

5 * 5 # yields 25

Lastly, expression statements

最后，表达式语句

print 5*5

回答by Rashid Iqbal

I think an expression contains operators + operands and the object that holds the result of the operation... e.g.

我认为表达式包含运算符 + 操作数和保存操作结果的对象......例如

var sum = a + b;

but a statement is simply a line of a code (it may be an expression) or block of code... e.g.

但语句只是一行代码（它可能是一个表达式）或代码块......例如

fun printHello(name: String?): Unit {
if (name != null)
    println("Hello ${name}")
else
    println("Hi there!")
// `return Unit` or `return` is optional

}

}

回答by ssokhey

Expressions:

表达式：

• Expressions are formed by combining objectsand operators.
• An expression has a value, which has a type.
• Syntax for a simple expression:<object><operator><object>

• 表达式由objects和组合而成operators。
• 一个表达式有一个值，它有一个类型。
• 简单表达式的语法：<object><operator><object>

2.0 + 3is an expression which evaluates to 5.0and has a type floatassociated with it.

2.0 + 3是一个表达式，其计算结果为5.0并具有float与之关联的类型。

Statements

声明

Statements are composed of expression(s). It can span multiple lines.

语句由表达式组成。它可以跨越多行。

回答by donald jiang

An expression is something, while a statement does something.
An expression is a statement as well, but it must have a return.

表达式是某物，而语句则是做某事。
表达式也是一个语句，但它必须有一个返回值。

>>> 2 * 2　　　　　　　  　#expression
>>> print(2 * 2)　　　　　#statement

PS:The interpreter always prints out the values of all expressions.

PS：解释器总是打印出所有表达式的值。