导入带有类定义的python模块时发生意外行为

Question

Consider the code in the file my_module.py:

class A(object):
  def __init__(self, x=er()):
    self.x = x

Now, when I import this module

import my_module

I get an error,

name 'er is not defined

While I understand that my_module does not have er defined, but I am never creating an instance of class A . Therefore it is puzzling that python tries to execute the __init__ callback when simply importing the module. Although, the __init__ call is not fully executed as explained by the example below:

class A(object):
  def __init__(self, x=5):
    self.x = x
    print ('I am here')

Now, when I import the module - nothing is printed and this is expected behavior.

I am puzzled why is function er called in the first example when I donot instantiate an object of class A . Any pointers to the documentation that explains this?

Answer 1

Because in Python, default argument values are evaluated at definition time . See, for example this question , or this notorious question .

This is documented here

The default values are evaluated at the point of function definition in the defining scope, so that

 i = 5 def f(arg=i): print(arg) i = 6 

f() will print 5.

Important warning : The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes. For example, the following function accumulates the arguments passed to it on subsequent calls:

 def f(a, L=[]): L.append(a) return L print(f(1)) print(f(2)) print(f(3)) 

This will print

 [1] [1, 2] [1, 2, 3] 

If you don't want the default to be shared between subsequent calls, you can write the function like this instead:

 def f(a, L=None): if L is None: L = [] L.append(a) return L