The First Python Program

Before formally writing our first Python program, let’s review what the command-line mode and Python interactive mode are.

Command-line Mode

In Windows, select “Terminal” from the Start menu to enter the PowerShell command-line mode, which uses a prompt similar to PS C:\>.

─────────────────────────────────────────────────────────┐
│Windows PowerShell                                 - □ x │
├─────────────────────────────────────────────────────────┤
│Windows PowerShell                                       │
│Copyright (C) Microsoft Corporation. All rights reserved.│
│                                                         │
│PS C:\Users\liaoxuefeng>                                 │
│                                                         │
└─────────────────────────────────────────────────────────┘

Python Interactive Mode

Type the command python in the command-line mode. You will see a series of text outputs and then enter the Python interactive mode, which uses the prompt >>>.

┌─────────────────────────────────────────────────────────┐
│Windows PowerShell                                 - □ x │
├─────────────────────────────────────────────────────────┤
│Windows PowerShell                                       │
│Copyright (C) Microsoft Corporation. All rights reserved.│
│                                                         │
│PS C:\Users\liaoxuefeng> python                          │
│Python 3.x ... on win32                                  │
│Type "help", ... for more information.                   │
│>>> _                                                    │
│                                                         │
└─────────────────────────────────────────────────────────┘

To exit the Python interactive mode and return to the command-line mode, type exit() and press Enter.

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│Windows PowerShell                                 - □ x │
├─────────────────────────────────────────────────────────┤
│Windows PowerShell                                       │
│Copyright (C) Microsoft Corporation. All rights reserved.│
│                                                         │
│PS C:\Users\liaoxuefeng> python                          │
│Python 3.x ... on win32                                  │
│Type "help", ... for more information.                   │
│>>> exit()                                               │
│                                                         │
│PS C:\Users\liaoxuefeng>                                 │
└─────────────────────────────────────────────────────────┘

You can also directly enter the Python interactive mode by selecting the “Python (command line)” menu item from the Start menu. However, after typing exit(), the window will close immediately and will not return to the command-line mode.

Now that we know how to start and exit Python’s interactive mode, we can officially begin writing Python code.

Before writing code, please do not use “copy”-“paste” to paste code from the page to your own computer. Writing programs also requires a sense of touch. You need to type the code letter by letter yourself. While typing, beginners often make mistakes: misspellings, incorrect capitalization, mixing Chinese and English punctuation, and mixing spaces and Tab keys. Therefore, you need to carefully check and compare to master how to write programs as quickly as possible.

In the interactive mode prompt >>>, enter code directly and press Enter to get the code execution result immediately. Now, try entering 100+200 and see if the calculation result is 300.

>>> 100+200
300

It’s very simple, any valid mathematical calculation can be worked out.

If you want Python to print specified text, you can use the print() function, and then enclose the text you want to print in single quotes or double quotes, but you cannot mix single quotes and double quotes.

>>> print('hello, world')
hello, world

Such text enclosed in single or double quotes is called a string in the program, and we will encounter it frequently in the future.

Finally, exit Python with exit(), and our first Python program is complete! The only drawback is that it is not saved, and you will have to re-enter the code next time you run it.

Command-line Mode vs. Python Interactive Mode

Please note the distinction between command-line mode and Python interactive mode.

In command-line mode, you can execute python to enter the Python interactive environment, or execute python hello.py to run a .py file.

Executing a .py file can only be done in command-line mode. If you type the command python hello.py and see the following error:

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├─────────────────────────────────────────────────────────┤
│Windows PowerShell                                       │
│Copyright (C) Microsoft Corporation. All rights reserved.│
│                                                         │
│PS C:\Users\liaoxuefeng> python hello.py                 │
│python: can't open file 'hello.py': [Errno 2] No such    │
│file or directory                                        │
│                                                         │
└─────────────────────────────────────────────────────────┘

The error message No such file or directory indicates that hello.py cannot be found in the current directory. You must first switch the current directory to the directory where hello.py is located before it can be executed normally.

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├─────────────────────────────────────────────────────────┤
│Windows PowerShell                                       │
│Copyright (C) Microsoft Corporation. All rights reserved.│
│                                                         │
│PS C:\Users\liaoxuefeng> cd work                         │
│PS C:\Users\liaoxuefengwork> python hello.py             │
│Hello, world!                                            │
│                                                         │
└─────────────────────────────────────────────────────────┘

The above commands assume that hello.py is stored in the work directory. If your hello.py is stored in another directory, such as the test directory, use cd test, or use an absolute path like cd \Users\who_am_i\learn\sample.

On Windows, if you want to switch to another drive letter, such as the D: drive, you need to enter D:.

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│Windows PowerShell                                 - □ x │
├─────────────────────────────────────────────────────────┤
│Windows PowerShell                                       │
│Copyright (C) Microsoft Corporation. All rights reserved.│
│PS C:\Users\liaoxuefeng> D:                              │
│PS D:\> cd work                                          │
│PS D:\work>                                              │
│                                                         │
└─────────────────────────────────────────────────────────┘

Under the D:\> prompt, continue to use the cd command to switch to the work directory containing the file hello.py, and then you can execute python hello.py normally.

In addition, running a .py file in command-line mode is different from running Python code directly in the Python interactive environment. The Python interactive environment automatically prints the result of each line of Python code, but running Python code directly does not.

For example, in the Python interactive environment, enter:

>>> 100 + 200 + 300
600

You can directly see the result 600.

However, create a file named calc.py with the following content:

100 + 200 + 300

Then execute it in command-line mode:

C:\work>python calc.py

You will find that there is no output.

This is normal. To output the result, you must print it yourself using print(). Modify calc.py as follows:

print(100 + 200 + 300)

Execute it again, and you will see the result:

C:\work>python calc.py
600

Finally, code in Python interactive mode is entered line by line and executed line by line, while directly running a .py file in command-line mode executes all code in the file at once. It can be seen that Python interactive mode is mainly used for debugging Python code and is also convenient for beginners to learn; it is not the environment for formally running Python code!

Syntax Error

If you encounter a SyntaxError, it means there is a syntax error in the entered Python code. One of the most common syntax errors is using Chinese punctuation, such as using Chinese parentheses （ and ）:

>>> print（'hello'）
  File "<stdin>", line 1
    print（'hello'）
         ^
SyntaxError: invalid character '（' (U+FF08)

Or using Chinese quotation marks “ and ”:

>>> print(“hello”)
  File "<stdin>", line 1
    print(“hello”)
          ^
SyntaxError: invalid character '“' (U+201C)

When an error occurs, be sure to read the error message. For the above SyntaxError, the interpreter will clearly indicate that the error is due to an unrecognizable character: invalid character '“'.

Summary

In Python interactive mode, you can enter code directly, then execute it, and get the result immediately.
In command-line mode, you can directly run .py files.

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