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How to understand the difference and use of Python modules, packages, many novices are not very clear about this, in order to help you solve this problem, the following editor will explain in detail for you, people with this need can come to learn, I hope you can gain something.

I. definition of modules and packages

Definition of module: any * .py file can be imported as a module using import

Definition of package: a directory containing a _ _ init__.py and other modules and other subpackages

What do the so-called packages and modules represent in the actual project, respectively, as follows:

A bag means test.

The module is do_excel.py,http_request.py,run.py.

II. Various methods of importing packages

Let's use the directory above to explain how to import packages in the run.py file.

Import a single test package

Import test

Import report, log packages

# Import at the same time, separated by commas, import report and log are recommended for wall cracks

# Import import reportimport log separately

Import a single test package and alias it

Import test as t

Import report and log packages and give them aliases

Import report as r, log as l

Import old package under project package

Import project.old

Import test package under old package under project package

# method 1: chain call import project.old.test

# method 2: from.. Import.. from project.old import test

Let's sum up the knowledge points here.

The package name followed by import is also the name of the unit variable (easy to understand by yourself)

What does it mean? Take the test package under the above old package as an example

Scenario: suppose I want to call the run () method in test.py under the test package

If you use method one, you have to write it this way.

Project.old.test.test.run ()

In the second way, it is written like this.

Test.test.run ()

You can see what "variable" is followed by import. When you want to call something in a package or module, you have to write "variable" before calling the package name and module name.

We can take a look at the following summary.

Import: full import

From.. Import..: partial import (targeted import)

How to distinguish the meaning of them? Let's take a look; the run.py code under the test package in the root directory is as follows

#! / usr/bin/env python

#-*-coding: utf-8-*-

Num = 1

Floats = 2.22

Strs = "string" def sum ():

Print ("I am sum")

Class test ():

Def test (self):

Print ("test class")

Suppose you want to call variables, function names, and class names under run.py in other files in the project

Import variables, function names and class names of other modules

Method 1: import mode

Everything in the run module (including variables, functions, class names) is imported, but it still has to be called unlinked through test.run

Import test.run# calls the num variable print (test.run.num) # output 1

# call the sum method print (test.run.sum ()) # output I am sum

Some friends just want to ask, can't I specify variables, function names, and class names? Let's try it.

#! / usr/bin/env python

#-*-coding: utf-8-*-

Import test.run.num

# call the num variable print (test.run.num)

First of all, if you are using Pycharm, the last num in import will be yellow, and there will be a prompt No module named num on hover.

Then, if you run the file, the error will be as follows

Traceback (most recent call last):

File "F:/test/tests.py", line 5, in

Import test.run.num,test.run.floats

ModuleNotFoundError: No module named 'test.run.num';' test.run' is not a package

The general meaning of the error is: test.run.num is not a module name, test.run is not a package

You can see that the complete import of import means that the smallest unit of import is the module, not the variable, function name, or class name.

Method 2: from.. Import..

# from package name. Module name import variable / function / class name from test.run import num from test.run import sumfrom test.run import testssfrom test.run import floats

If _ _ name__ = ='_ _ main__':

Print (num)

Print (sum ())

Print (testss () .test ())

Print (floats)

Execution result

one

I'm sum.

Test class

2.22

You can see from.. Import.. The import minimum unit can be a module, a variable, a function name, or a class name

Fourth, the method of importing module

According to the knowledge mentioned above, there are two ways to import a module, one is the import module name and the other is the from package name import module name

Import the run module under the test package

# importimport test.run

# fromfrom run import test

Import run and tests modules under the test package

# importimport test.run,test.tests

# fromfrom run import test,tests

Special knowledge

Question: where are import packages / modules, packages and modules imported from?

Answer: we searched in order through the list of paths included in sys.path.

How do you understand it?

Click the following code under the Pycharm project

Import sysprint (sys.path)

Execution result

['F:\\ moocInterface\\ test', 'F:\\ moocInterface', 'D:\\ PyCharm 2019.3.1\\ plugins\\ python\\ helpers\\ pycharm_display', 'D:\\ python3.6\\ python36.zip', 'D:\\ python3.6\\ DLLs', 'D:\\ python3.6\\ lib', 'D:\\ python3.6', 'D:\\ python3.6\\ lib\\ site-packages' 'D:\\ python3.6\\ lib\\ site-packages\\ django-2.1.5-py3.6.egg','D:\\ python3.6\\ lib\\ site-packages\\ pytz-2018.9-py3.6.egg','D:\\ PyCharm 2019.3.1\\ plugins\\ python\\ helpers\\ pycharm_matplotlib_backend']

You can see:

1. The path of the current file is placed first.

two。 The engineering path is in the second place.

3.Pycharm related paths are placed in the third place.

We enter the Python interpreter from the system cmd and type the following code to see the result

> import sys > print (sys.path)

The results are as follows

['','D:\\ python3.6\\ python36.zip', 'D:\\ python3.6\\ DLLs', 'D:\\ python3.6\\ lib', 'D:\\ python3.6', 'D:\\ python3.6\\ lib\\ site-packages', 'D:\\ python3.6\\ lib\\ site-packages\\ django-2.1.5-py3.6.egg' 'd:\\ python3.6\\ lib\\ site-packages\\ pytz-2018.9-py3.6.egg']

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