Numpy Referenced before assignment error in python

Question

here is my python code:

import numpy
class test:
  def __init__(self,):
    print(numpy.__version__)
    if False:
      numpy = None

if __name__=='__main__':
  print(numpy.__version__)
  T = test()

when I run this code, the interpreter give me an error showed blow:

UnboundLocalError: local variable 'numpy' referenced before assignment

It seems like, before executing numpy = None, imported module "numpy" has been covered while there is no numpy variable.

My question is what exactly the interpreter did during initializing a class(not object)?

Why are you assigning numpy = None? This makes Python think numpy is a local variable. — tobias_k
– tobias_k, Commented Sep 2, 2020 at 10:23
If your function contains an assignment to a variable, then by default the variable is local to the function. — khelwood
– khelwood, Commented Sep 2, 2020 at 10:26

Safwan Samsudeen · Accepted Answer · 2020-09-03 03:39:41Z

1

EDIT: As a reply to your comment, Python goes through each line of code before executing it, and sees that the syntax is correct and other things. To test it, run the following code.

def foo():
    print("Hello, I won't be printed.")
    : # Syntax Error!!!
foo()

It will raise a SyntaxError without printing anything even though the SyntaxError is after the print statement, because it checks the code and then only runs it.

Inside a function, it also thinks that all the variables which are found to be assigned while checking are supposed to be local. If you don't want this, then you must explicitly say so using the global or nonlocal keywords.

class test:
  def __init__(self,):
    global numpy
    print(numpy.__version__)
    if False:
      numpy = None

edited Sep 3, 2020 at 3:39

answered Sep 2, 2020 at 10:33

Safwan Samsudeen

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7 Comments

Safwan Samsudeen Over a year ago

@Asocia, Done. What does OP stand for?

Humble_me Over a year ago

The code is meaningless, just for reproducing the issue I met. But I am confused why the variable is local before excuating numpy = None

Humble_me Over a year ago

The codes are excuated line by line, so why numpy = None will influence the previous ussage of numpy? What did python interpreter do to the class before initializing it?

Humble_me Over a year ago

thx for your answer, it works. It seems like a rule of python, so why don't python convert numpy to local variable until assignment executed?

Humble_me Over a year ago

almost all the variables can be covered while assigning, why global variable cannot be covered by local variable? It is a technical limit or a thoughtful design?

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