python函数参数改不改变的问题

python函数参数改不改变的问题

**结论：**python有可变对象和不可变对象之分。如果传入的参数是不可变对象，则在函数体内对形参的修改不会导致实参被修改，而如果传入的是可变对象，实参有可能会变，也有可能不变，这取决于进行改变的操作。

• 不可变对象 : Number,String,Tuple,bool
• 可变对象 ： List，Set，Dictionary

1.不可变对象

def test(str1):
    str1 = "inside"
    print("this is function "+str1)
str2 = "outside"
print("this is function "+str2)
test(str2)
print("this is function "+str2)     # str2的值并没有被改变

运行结果：

this is function outside
this is function inside
this is function outside

对于其他不可变对象，结果一致。

2.可变对象的改变操作

def test(str1):
    str1.append(3)
    print("this is function,the value is ",str1)

myStr = [1,2]
print("this is function outside ,the value is ",myStr)
test(myStr)
print("this is function outside ,the value is ",myStr)

运行结果：

this is function outside ,the value is [1, 2]
this is function,the value is [1, 2, 3]
this is function outside ,the value is [1, 2, 3]

python中函数的形参，传入的是实参的引用，实质是指向实参的地址，所以对于形参的操作会改变实参的指。

3.可变对象的赋值改变操作

除了使用python的自带函数，也可通过赋值来修改实参的值：

def test(str1):
    for i in range(len(str1)):
        str1[i] = i+4
    print("this is function,the value is ",str1)

myStr = [1,2]
print("this is function outside ,the value is ",myStr)
test(myStr)
print("this is function outside ,the value is ",myStr)

运行结果：

this is function outside ,the value is [1, 2]
this is function,the value is [4, 5]
this is function outside ,the value is [4, 5]

4.可变对象的不可变操作

def test(str1):
    str1 = [1,2,3]
    print("this is function,the value is ",str1)

myStr = [1,2]
print("this is function outside ,the value is ",myStr)
test(myStr)
print("this is function outside ,the value is ",myStr)

运行结果：

this is function outside ,the value is [1, 2]
this is function,the value is [1, 2, 3]
this is function outside ,the value is [1, 2]

正如上文所说，形参指向的是实参的地址，**当形参被重新赋值时，如果重新赋值后的对象所占的空间大小没有改变，则可以实现对实参的改变。**即对可变对象不改变长度 len()，则可以实现改变。

如果赋值后，形参的长度已经发生了改变，则python会创建一个新的变量，并把变量地址给形参，这时形参和实参不再指向同一个地址了，对形参的修改与实参毫无关系了。

验证如下：

def test(str1):
    str1 = [1]
    print("this is function,the value is ",str1)

myStr = [1,2]
print("this is function outside ,the value is ",myStr)
test(myStr)
print("this is function outside ,the value is ",myStr)

运行结果：

this is function outside ,the value is [1, 2]
this is function,the value is [1]
this is function outside ,the value is [1, 2]

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