Shulou(Shulou.com)06/02 Report--

This article shows you how to solve the problem of mutual introduction of Python. The content is concise and easy to understand. It will definitely brighten your eyes. I hope you can get something through the detailed introduction of this article.

Problem description

A series of errors occurred when the two files were imported into each other, such as ImportError, NameError, and so on. This time, I'll recreate these problems with simple code examples and solve them step by step.

Problem recurrence

Here, I create two Python files in the same directory: mutualImportA and mutualImportB, and enter the code.

# mutualImportA.py from mutualImportB import b a = 1 def pA (): print (b) pA () # mutualImportB.py from mutualImportA import a b = 1 def pB (): print (a) pB () after creation, any run An or B will report an error: ImportError: cannot import name 'b'from partially initialized module 'mutualImportB' (most likely due to a circular import) of course some people are used to using import import instead of from import import. At this point, the error report will become: AttributeError: partially initialized module 'mutualImportB' has no attribute' b'(most likely due to a circular import)

Okay, these two mistakes are pretty friendly. This method of import indicates the content of the import in the program, so the interpreter can find that this is a circular most likely due to a circular import. But sometimes it's the NameError that gives you a real headache.

Let's change the code to keep the from import statement, but change the variable name after its import to the wildcard character *. This time the error report:

NameError: name 'b'is not defined

That's weird. When we use IDE, IDE does not detect circular import problems caused by mutual imports. However, the first two can be seen in the error report, and at this time, it is difficult to find out why b is not declared. This kind of small file problems that appear specifically for reproduction can clarify the train of thought and find the problem, but there is often no way to start in larger projects.

So why did this happen? Let's think about the running process of the whole program.

After the recurrence of the problem, let's discuss how to solve the problem.

Solve the problem

I have seen articles on many platforms before, but most of them are completely repetitive (especially on CSDN), obviously "reprinting" each other. Unfortunately, reprinted articles can not really solve the problem. In this original article, I share my experience of groping. Let's get down to business.

What is the root cause of circular import? Why is there a circular import?

In fact, by loop import, we can think of stack overflow caused by recursion.

Def a (b): return a (b) this is an obviously wrong function. Once we have called the function, the function will be called again inside the function without judgment. When we add judgment: def a (b): if b = = 1: return 1 else: return a (b-1), the function is transformed into a healthy function.

Apply the same idea to the way of introducing each other. Do we have to import a module at the beginning of the whole file? Generally speaking, not. We should put the import in the function it really needs. For example, change the an and b in the recurrence of the problem:

# mutualImportA.py a = 1 def pA (): from mutualImportB import b print (b) pA () # mutualImportB.py b = 1 def pB (): from mutualImportA import a print (a) pB (), there is no problem with the loop import above. This time, divide the process again:

OK, then the problem is solved.

Of course, this problem also gives rise to some code specification problems:

In development, if the file is small, it is still best not to divide the two functions or classes that need to use each other into two files. In this way, many problems can be avoided.

The above content is how to solve the problem of mutual import of Python. Have you learned the knowledge or skills? If you want to learn more skills or enrich your knowledge reserve, you are welcome to follow the industry information channel.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.