子类_​​_init__看不到超类按条件导入的模块

Question

I have a conditional import in a self-initialized instance of a superclass, but subclass cannot see the module (python 2.7):

class A(object):
    def __init__(self, arg1):
        self.attr1 = self.method1(arg1)

    def method1(self, arg1):
        if arg1 == 'foo':
           import amodule
           return amodule.method1()
        else:
            return 'not a dependency on foo'


class B(A):
    def __init__(self, arg1):
        super(B, self).__init__(arg1)
        if arg1 == 'foo':
            self.attr2 = self.method2(self.attr1)

    def method2(self, attr1):
        return amodule.method2()

if __name__=='__main__':
    b = B("foo")
    print b.attr2

This throws NameError: global name 'amodule' is not defined . a = A("foo") works just fine

Shouldn't the super call have executed import amodule in this case? (And using import should have put the module into globals?)

Answer 1

Doesn't import add /amodule/ to the global namespace of the currently executing module? ( __main__ )?

No, the module is added to sys.modules but if it was imported locally then you won't have any references to it anymore. ie the name amodule is now gone.

You could still access the module using sys.modules :

def method2(self, attr1):
    import sys
    return sys.modules['amodule'].method2()

Or you could import it using import amodule again and it will be picked up from sys.modules .

---

# Here b.py contains
# print('Module b was imported')

def func1():
    print('inside func1')
    import b

def func2():
    print('inside func2')
    import sys
    print(sys.modules['b'])
    import b


def func3():
    print('inside func3')
    import b
    import sys
    print('Deleted b')
    del sys.modules['b']
    import b


func1()
print()
func2()
print()
func3()

Demo:

inside func1
Module b was imported

inside func2
<module 'b' from '/Users/ashwini/py/b.py'>

inside func3
Deleted b
Module b was imported

Answer 2

Try putting import amodule on the first line of the program.

The reason is that amodule is imported in method1, method2 does not have access.

Answer 3

If you follow your code, you would see you don't reach method1().

When you create the object b = B(foo) You go through A. init () becuase the call for super(). However, your init of class A() doesn't include any import statements. Then the A.__init__ part is done, and you continue with B.__init__() . The next command is a call to amodule object, which wasn't imported at all.

You can add an helper method, which checks if the arg equals 'Foo' and if so import the module. Then add a call to this function in A.__init__() function.

On another note, __init__() job is to initialize variables. It shouldn't return anything.