Python ModuleNotFoundError与ImportError排查

创建时间：2021/5/18 17:17

【Python】How to Fix ModuleNotFoundError and Im

• 【Python】How to Fix ModuleNotFoundError and ImportError
  • Terminology
  • How does module import work behind the scenes?
    • Step 1: sys.modules lookup
    • Step 2: Python Standard Library lookup
    • Step 3: sys.path lookup
  • Absolute vs Relative imports
    • absolute imports
    • relative imports
  • Resolution
    • Use IDE
    • Other cases
      • 方法一：命令行
      • 方法二：python scripts
  • Ref

Terminology

python module - a single file with a .py extension. python package - a folder that contains at least one python module.

• For python2, a package requires a init.py file

• A python package can contain any number of nested sub-packages

string str1 = String.Empty;
string str2 = String.Empty;

str2 = String.Intern(sb.ToString());

if((object)str1==(object)str2)
    Console.WriteLine("The strings are equal.");
else
    Console.WriteLine("The strings are not equal.");

Project myproject contains two packages, mypackage and anotherpackage each of which contains a number of python modules, while the latter also contains a sub-package called mysubpackage which in turn contains an additional python module.

How does module import work behind the scenes?

string str1 = String.Empty;
string str2 = String.Empty;

str2 = String.Intern(sb.ToString());

if((object)str1==(object)str2)
    Console.WriteLine("The strings are equal.");
else
    Console.WriteLine("The strings are not equal.");

Python will execute the above statement in two steps:

• Locate, load and initialise (if required) the requested module

• Define necessary names in the local namespace and corresponding scope

Now Python interpreter is going to follow the next steps in an attempt to resolve a .

Step 1: sys.modules lookup

Initially, Python will try to search for the module’s name in sys.modules , which is a dictionary that maps module names to modules which have already been loaded. If the name is resolved successfully (which means that another module has already loaded) it will be then be made available to the local namespace otherwise, jump into step 2.

string str1 = String.Empty;
string str2 = String.Empty;

str2 = String.Intern(sb.ToString());

if((object)str1==(object)str2)
    Console.WriteLine("The strings are equal.");
else
    Console.WriteLine("The strings are not equal.");

Step 2: Python Standard Library lookup

Python Standard Library contains built-in modules (written in C) that provide access to system functionality such as file I/O that would otherwise be inaccessible to Python programmers, as well as modules written in Python that provide standardized solutions for many problems that occur in everyday programming. Some of these modules are explicitly designed to encourage and enhance the portability of Python programs by abstracting away platform-specifics into platform-neutral APIs.

If the name couldn’t be found in sys.modules then Python is going to search for it in Python Standard Library. Again, if the name is resolved then it will be defined in the local namespace otherwise step 3 needs to be followed.

Step 3: sys.path lookup

Now if the module’s name was not found either in sys.modules nor in standard library, Python will finally attempt to resolve it under sys.path.

Absolute vs Relative imports

We discourage the use of relative imports as they are not as readable as absolute imports

string str1 = String.Empty;
string str2 = String.Empty;

str2 = String.Intern(sb.ToString());

if((object)str1==(object)str2)
    Console.WriteLine("The strings are equal.");
else
    Console.WriteLine("The strings are not equal.");

absolute imports

string str1 = String.Empty;
string str2 = String.Empty;

str2 = String.Intern(sb.ToString());

if((object)str1==(object)str2)
    Console.WriteLine("The strings are equal.");
else
    Console.WriteLine("The strings are not equal.");

relative imports

string str1 = String.Empty;
string str2 = String.Empty;

str2 = String.Intern(sb.ToString());

if((object)str1==(object)str2)
    Console.WriteLine("The strings are equal.");
else
    Console.WriteLine("The strings are not equal.");

Resolution

核心原则：

• use absolute imports

• append your project’s root directory to PYTHONPATH

本质是：

• 给sys.path中添加路径，使得import时能够找到相应的module。

• 给sys.modules中添加item

Use IDE

Most modern Python IDEs will do the trick automatically but if this is not the case, I am pretty sure there will be such option where you’ll be able to define the PYTHONPATH for your Python application

Other cases

以下两种方法效果相同。

方法一：命令行

If you are running your Python application in any other environment such as Docker , Vagrant or inside your virutal environment you can run the below command in your bash:

string str1 = String.Empty;
string str2 = String.Empty;

str2 = String.Intern(sb.ToString());

if((object)str1==(object)str2)
    Console.WriteLine("The strings are equal.");
else
    Console.WriteLine("The strings are not equal.");

上述命令会将相应的path添加到 sys.path 当中。

方法二：python scripts

Add this code in your python scripts:

string str1 = String.Empty;
string str2 = String.Empty;

str2 = String.Intern(sb.ToString());

if((object)str1==(object)str2)
    Console.WriteLine("The strings are equal.");
else
    Console.WriteLine("The strings are not equal.");

Ref

https://towardsdatascience.com/how-to-fix-modulenotfounderror-and-importerror-248ce5b69b1c