Python中相对路径（导入）和绝对路径（导入）的区别

　　参考资料：https://realpython.com/absolute-vs-relative-python-imports/#relative-imports

Absolute Imports

You’ve gotten up to speed on how to write import statements and how to style them like a pro. Now it’s time to learn a little more about absolute imports.

An absolute import specifies the resource to be imported using its full path from the project’s root folder.

Syntax and Practical Examples

Let’s say you have the following directory structure:

└── project
    ├── package1
    │   ├── module1.py
    │   └── module2.py
    └── package2
        ├── __init__.py
        ├── module3.py
        ├── module4.py
        └── subpackage1
            └── module5.py

There’s a directory, project, which contains two sub-directories, package1 and package2. The package1 directory has two files, module1.py and module2.py.

The package2 directory has three files: two modules, module3.py and module4.py, and an initialization file, __init__.py. It also contains a directory, subpackage, which in turn contains a file, module5.py.

Let’s assume the following:

1. package1/module2.py contains a function, function1.
2. package2/__init__.py contains a class, class1.
3. package2/subpackage1/module5.py contains a function, function2.

The following are practical examples of absolute imports:

from package1 import module1
from package1.module2 import function1
from package2 import class1
from package2.subpackage1.module5 import function2

Note that you must give a detailed path for each package or file, from the top-level package folder. This is somewhat similar to its file path, but we use a dot (.) instead of a slash (/).

 

Pros and Cons of Absolute Imports

Absolute imports are preferred because they are quite clear and straightforward. It is easy to tell exactly where the imported resource is, just by looking at the statement. Additionally, absolute imports remain valid even if the current location of the import statement changes. In fact, PEP 8 explicitly recommends absolute imports.

Sometimes, however, absolute imports can get quite verbose, depending on the complexity of the directory structure. Imagine having a statement like this:

from package1.subpackage2.subpackage3.subpackage4.module5 import function6

That’s ridiculous, right? Luckily, relative imports are a good alternative in such cases!

Relative Imports

A relative import specifies the resource to be imported relative to the current location—that is, the location where the import statement is. There are two types of relative imports: implicit and explicit. Implicit relative imports have been deprecated in Python 3, so I won’t be covering them here.

Syntax and Practical Examples

The syntax of a relative import depends on the current location as well as the location of the module, package, or object to be imported. Here are a few examples of relative imports:

from .some_module import some_class
from ..some_package import some_function
from . import some_class

You can see that there is at least one dot in each import statement above. Relative imports make use of dot notation to specify location.

A single dot means that the module or package referenced is in the same directory as the current location. Two dots mean that it is in the parent directory of the current location—that is, the directory above. Three dots mean that it is in the grandparent directory, and so on. This will probably be familiar to you if you use a Unix-like operating system!

Let’s assume you have the same directory structure as before:

└── project
    ├── package1
    │   ├── module1.py
    │   └── module2.py
    └── package2
        ├── __init__.py
        ├── module3.py
        ├── module4.py
        └── subpackage1
            └── module5.py

Recall the file contents:

1. package1/module2.py contains a function, function1.
2. package2/__init__.py contains a class, class1.
3. package2/subpackage1/module5.py contains a function, function2.

You can import function1 into the package1/module1.py file this way:

# package1/module1.py

from .module2 import function1

You’d use only one dot here because module2.py is in the same directory as the current module, which is module1.py.

You can import class1 and function2 into the package2/module3.py file this way:

# package2/module3.py

from . import class1
from .subpackage1.module5 import function2

In the first import statement, the single dot means that you are importing class1 from the current package. Remember that importing a package essentially imports the package’s __init__.py file as a module.

In the second import statement, you’d use a single dot again because subpackage1 is in the same directory as the current module, which is module3.py.