python class inheritance order

Question

I have a file test.py

class A(B):
  def display(self):
      print ("In A")

class B:
  def display(self):
    print ("In B")

I get the following error while run it Traceback (most recent call last):

File "/Users/praveen/Documents/test.py", line 1, in <module>
   class A(B):
NameError: name 'B' is not defined

But if I change the order of declaration, It runs without any errors

class B:
  def display(self):
    print ("In B")

class A(B):
  def display(self):
      print ("In A")

Can anyone explain in detail why this weird error happens?

Because Python won't know what B means until you've defined it. It's not really that weird. — khelwood
– khelwood, Commented Jan 24, 2018 at 9:54

FERNman · Accepted Answer · 2018-01-24 09:56:34Z

3

This happens because python gets interpreted Top-To-Bottom. In the line where you define class A(B) in your first example, class B was not yet read by python.

In your second example, B is already known in the line class A(B). That's why it runs.

answered Jan 24, 2018 at 9:56

FERNman

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AntiMatterDynamite · Accepted Answer · 2018-01-24 09:58:45Z

2

simple: when python evaluates class A(B): B is still undefined,

unfortunately python has no class prototypes (or forward declarations)

but this is only a problem if you have 2 classes that explicitly need to point to each other.

answered Jan 24, 2018 at 9:58

AntiMatterDynamite

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