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The>>>that you see on the last line is the Python interactive prompt, which I will refer to simply as the prompt.
Before we go any further, I need to introduce you to the concept of program comments .
In programming jargon, a comment is text that you insert into the program that is intended for human consumption only. Comments provide a quick and easyform of documentation. They are ignored by the computer and are intended to explain what you are doing.
Comments aren't terribly useful when doing interactive programming. Presumably you already know what you are doing and don't need to explain it toyourself using comments.
However, comments are very useful when you are writing scripts that you will store in files and use again later after you have forgotten how you did what youdid.
In these modules, I will use comments occasionally to explain what I am doing for your benefit, even in interactive mode.
According to the Python Language Reference -- 2.1.3 Comments :
"A comment starts with a hash character (#) that is not part of a string literal, and ends at the end of the physical line.A comment signifies the end of the logical line unless the implicit line joining rules are invoked. Comments are ignored by the syntax; they are not tokens."
Figure 2 shows a Python comment, taken from the Python command-line interface, along with a new kind of prompt.
| Figure 2 . A Python comment. |
|---|
>>># This is a comment
... |
The interactive mode actually uses two different kinds of prompts.
I will explain the difference a little later For now, just pretend like they both mean the same thing. That will suffice until we get into morecomplicated material.
Enter 2+5 at the prompt in the IDLE GUI interface and press the Enter key. You should see something like Figure 3 on your screen.
| Figure 3 . The sum of 2 and 5. |
|---|
>>># This is a comment in Python GUI shell>>>2+5
7>>> |
We need to differentiate between input and output. Everything that appears on a line with one of the prompts is input to the Pythoninterpreter. (You typed it, so you know that it is input.)
Everything that appears on a line without a prompt is output from the interpreter. (You didn't type it. The interpreter produced it, so it was output.)
Your input was the expression 2+5.
Python evaluated that expression and produced an output, which was the sum of 2 and 5, or 7.
Then Python presented you with a new prompt to allow you to provide more input.
Now using the Python command-line window, try entering the comment, followed by the expression that you see in Figure 4 , and note the difference in the prompts as compared to Figure 3 ..
| Figure 4 . Mixing comments and expressions on the command line interface. |
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>>># This is a comment in Python command-line interface
... 2+57>>> |
My only reason for showing you this at this time is to give you some experience in mixing comments and expressions and to attempt to explain thedifference between the>>>prompt and the ... prompt.
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