Python 为什么exec()中的符号定义语句有时对本地符号表没有影响?
以下代码段按预期工作:Python 为什么exec()中的符号定义语句有时对本地符号表没有影响?,python,python-3.x,exec,local-variables,symbol-table,Python,Python 3.x,Exec,Local Variables,Symbol Table,以下代码段按预期工作: def test(): print(f'local symbol table before exec : {locals()}') exec('a = 0') print(f'local symbol table after exec : {locals()}') test() # printed result: # local symbol table before exec : {} # local symbol table after e
def test():
print(f'local symbol table before exec : {locals()}')
exec('a = 0')
print(f'local symbol table after exec : {locals()}')
test()
# printed result:
# local symbol table before exec : {}
# local symbol table after exec : {'a': 0}
但是,一旦我在test
函数末尾添加了符号定义语句a=1
,似乎exec
语句对本地符号表没有影响:
def test():
print(f'local symbol table before exec : {locals()}')
exec('a = 0')
print(f'local symbol table after exec : {locals()}')
a = 1
test()
# printed result:
# local symbol table before exec : {}
# local symbol table after exec : {}
那么,为什么会发生这种情况?
我的猜测是:函数中静态定义的符号将在编译时以某种方式保留,如果已经保留了符号,则在exec函数中动态调用的任何符号定义语句都无法修改本地符号表
这是真的吗?编译期间实际发生了什么
额外测试1:将exec参数替换为
'a=0\n点(locals())'
正如我们所看到的,符号a
在exec()
执行期间成功地添加到本地符号表中,但在执行之后,随着a=1
的存在,它立即神奇地消失了
额外测试2:在
a=1之前添加return
语句
def test():
print(f'local symbol table before exec : {locals()}')
exec('a = 0\nprint(locals())')
print(f'local symbol table after exec : {locals()}')
return
test()
# printed result:
# local symbol table before exec : {}
# {'a': 0}
# local symbol table after exec : {'a': 0}
a=1
在第二个test()
函数中不可访问,但它仍会影响exec()
的行为
即使是dis
模块中的dis()
函数也无法区分这两个test()
函数之间的区别。输出完全相同,如下所示:
5 0 LOAD_GLOBAL 0 (print)
2 LOAD_CONST 1 ('local symbol table before exec : ')
4 LOAD_GLOBAL 1 (locals)
6 CALL_FUNCTION 0
8 FORMAT_VALUE 0
10 BUILD_STRING 2
12 CALL_FUNCTION 1
14 POP_TOP
6 16 LOAD_GLOBAL 2 (exec)
18 LOAD_CONST 2 ('a = 0\nprint(locals())')
20 CALL_FUNCTION 1
22 POP_TOP
7 24 LOAD_GLOBAL 0 (print)
26 LOAD_CONST 3 ('local symbol table after exec : ')
28 LOAD_GLOBAL 1 (locals)
30 CALL_FUNCTION 0
32 FORMAT_VALUE 0
34 BUILD_STRING 2
36 CALL_FUNCTION 1
38 POP_TOP
8 40 LOAD_CONST 0 (None)
42 RETURN_VALUE
根据报告:
注意:默认局部变量的作用如下面函数locals()所述:不应尝试修改默认局部变量字典传递一个显式局部变量
如果需要在函数exec()返回后查看代码对局部变量的影响,请使用字典。
因此,我相信这属于“意外行为”,但我想您可以转到exec()
的实现来真正深入了解
def test():
print(f'local symbol table before exec : {locals()}')
exec('a = 0\nprint(locals())')
print(f'local symbol table after exec : {locals()}')
return
test()
# printed result:
# local symbol table before exec : {}
# {'a': 0}
# local symbol table after exec : {'a': 0}
def test():
print(f'local symbol table before exec : {locals()}')
exec('a = 0\nprint(locals())')
print(f'local symbol table after exec : {locals()}')
return
a = 1
test()
# printed result:
# local symbol table before exec : {}
# {'a': 0}
# local symbol table after exec : {}
5 0 LOAD_GLOBAL 0 (print)
2 LOAD_CONST 1 ('local symbol table before exec : ')
4 LOAD_GLOBAL 1 (locals)
6 CALL_FUNCTION 0
8 FORMAT_VALUE 0
10 BUILD_STRING 2
12 CALL_FUNCTION 1
14 POP_TOP
6 16 LOAD_GLOBAL 2 (exec)
18 LOAD_CONST 2 ('a = 0\nprint(locals())')
20 CALL_FUNCTION 1
22 POP_TOP
7 24 LOAD_GLOBAL 0 (print)
26 LOAD_CONST 3 ('local symbol table after exec : ')
28 LOAD_GLOBAL 1 (locals)
30 CALL_FUNCTION 0
32 FORMAT_VALUE 0
34 BUILD_STRING 2
36 CALL_FUNCTION 1
38 POP_TOP
8 40 LOAD_CONST 0 (None)
42 RETURN_VALUE