NameError：未定义名称“类”

Question

When I compile I get this error:

Traceback (most recent call last):
  File "c:/Users/dvdpd/Desktop/ProjectStage/Main.py", line 1, in <module>
    class Main:
  File "c:/Users/dvdpd/Desktop/ProjectStage/Main.py", line 6, in Main
    test = Reading()
NameError: name 'Reading' is not defined

Code:

class Main:
    print("Welcome.\n\n")
    test = Reading()
    print(test.openFile)


class Reading:
    def __init__(self):
        pass

    def openFile(self):
        f = open('c:/Users/dvdpd/Desktop/Example.txt')
        print(f.readline())
        f.close()

I can't use the class Reading and I don't know why. Main and Reading are in the same file so I think I don't need an import .

Answer 1

Python source files are interpreted top to bottom by the interpreter.

So, when you call Reading() inside class Main , it does not exist yet. You need to swap the declarations to put Reading before Main .

Answer 2

您需要在Main之前定义Reading

Answer 3

You need to define class Reading before class Main .

class Reading:
    def __init__(self):
        pass

    def openFile(self):
        f = open('c:/Users/dvdpd/Desktop/Example.txt')
        print(f.readline())
        f.close()


class Main:
    print("Welcome.\n\n")
    test = Reading()
    print(test.openFile())

Answer 4

Forward declaration doesn't work in Python. So you'll get an error only if you create an object of the Main class as follows:

class Main:
    def __init__(self):
        print("Welcome.\n\n")
        test = Reading()
        print(test.openFile)

# Main() # This will NOT work

class Reading:
    def __init__(self):
        pass

    def openFile(self):
        f = open('c:/Users/dvdpd/Desktop/Example.txt')
        print(f.readline())
        f.close()

# Main() # This WILL work