53 lines
1.9 KiB
Markdown
53 lines
1.9 KiB
Markdown
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---
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title: Python Calling Functions
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localeTitle: Python调用函数
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---
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函数定义语句不执行该函数。执行（调用）函数是通过使用函数的名称，后跟括起所需参数（如果有）的括号来完成的。
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```
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>>> def say_hello():
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... print('Hello')
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...
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>>> say_hello()
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Hello
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```
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函数的执行引入了用于函数局部变量的新符号表。更准确地说，函数中的所有变量赋值都将值存储在本地符号表中;而变量引用首先在本地符号表中查找，然后在封闭函数的本地符号表中查找，然后在全局符号表中查找，最后在内置名称表中查找。因此，全局变量不能直接在函数内赋值（除非在全局语句中命名），尽管可以引用它们。
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```
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>>> a = 1
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>>> b = 10
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>>> def fn():
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... print(a) # local a is not assigned, no enclosing function, global a referenced.
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... b = 20 # local b is assigned in the local symbol table for the function.
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... print(b) # local b is referenced.
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...
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>>> fn()
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1
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20
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>>> b # global b is not changed by the function call.
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10
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```
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调用函数调用的实际参数（参数）在被调用函数的本地符号表中引入;因此，使用call by value传递参数（其中值始终是对象引用，而不是对象的值）。当函数调用另一个函数时，将为该调用创建一个新的本地符号表。
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```
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>>> def greet(s):
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... s = "Hello " + s # s in local symbol table is reassigned.
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... print(s)
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...
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>>> person = "Bob"
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>>> greet(person)
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Hello Bob
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>>> person # person used to call remains bound to original object, 'Bob'.
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'Bob'
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```
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用于调用函数的参数不能由函数重新分配，但引用可变对象的参数可以更改其值：
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```
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>>> def fn(arg):
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... arg.append(1)
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...
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>>> a = [1, 2, 3]
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>>> fn(a)
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>>> a
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[1, 2, 3, 1]
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```
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