Python 3 interpreter

On Linux/Unix systems, Python interpreters are usually installed in valid paths (directories) such as /usr/local/python3.

We can add the path /usr/local/python3/bin to the environment variables of your Linux/Unix operating system (preferably with reference to your Python installation path) so that you can start Python by entering the following command at the shell terminal.

$ PATH=$PATH:/user/local/python3/bin/python #设置环境变量
$ python3 --vesion
Python 3.7.6

Under the Window system, you can set Python's environment variables with the following commands, assuming that your Python is installed under C:

set path=%path%;C:\python37

Interactive programming

When IDLE is turned on, the default is interactive programming, similar to a cmd window.

We can start the Python interpreter by entering the "Python" command in the command prompt:

$ python3

After executing the above command, the following window message appears:

$ python3
Python 3.4。0 (default, Mar 16 2014, 09:25:04)
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Enter the following statement in the Python prompt, and then press enter to see how it works:

print ("Welcome to W3Cschool!");

The above commands are executed as follows:

When you type a multi-line structure, a continued line is a must. Let's look at the following if statement:

>>> the_world_is_flat = True
>>> if the_world_is_flat:
...     print("Be careful not to fall off!")
...
Be careful not to fall off!

Scripted programming

Click file → new script file, under which scripting is carried out.

Copy the following code to W3C.py file:

print ("Welcome to W3Cschool!");

The script is executed with the following command:

python W3C.py

The output is:

Welcome to W3Cschool!

In a Linux/Unix system, you can add the following commands at the top of the script so that the Python script can execute as directly as the SHELL script:

#! /usr/bin/env python3.7

Then modify the script permissions so that they have execution permissions, and the command is as follows:

$ chmod +x W3C.py

Follow these commands:

./hello.py

The output is:

Welcome to W3Cschool!