Why use Python interactive mode?

Let's see:

• If you want to know how something works, you can just try it. There is no need to write up a file. I almost always scratch write my programs in the interpreter before coding them. It's not just for things that you don't know how they work in the programming language. I never remember what the correct arguments to range are to create, for example, [-2, -1, 0, 1]. I don't need to. I just have to fire up the interpreter and try stuff until I figure out it is range(-2, 2) (did that just now, actually).

• You can use it as a calculator.

• Python is a very introspective programming language. If you want to know anything about an object, you can just do dir(object). If you use IPython, you can even do object.<TAB> and it will tab-complete the methods and attributes of that object. That's way faster than looking stuff up in documentation or even in code.

• help(anything) for documentation. It's way faster than any web interface.

• Again, you have to use IPython (highly recommended), but you can time stuff. %timeit func1() and %timeit func2() is a common idiom to determine what is faster.

• How often have you wanted to write a program to use once, and then never again. The fastest way to do this is to just do it in the Python interpreter. Sure, you have to be careful writing loops or functions (they must have the correct syntax the first time), but most stuff is just line by line, and you can play around with it.

• Debugging. You don't need to put selective print statements in code to see what variables are when you write it in the interpreter. You just have to type >>> a, and it will show what a is. Nice again to see if you constructed something correctly. The building Python debugger pdb also uses the intrepeter functionality, so you can not only see what a variable is when debugging, but you can also manipulate or even change it without halting debugging.

When people say that Python is faster to develop in, I guarantee that this is a big part of what they are talking about.

Commenters: anything I am forgetting?

REPL Loops (like Python's interactive mode) provide immediate feedback to the programmer. As such, you can rapidly write and test small pieces of code, and assemble those pieces into a larger program.

You're talking about running Python in the console by simply typing "python"? That's just for little tests and for practicing with the language. It's very useful when learning the language and testing out other modules.

Of course any real software project is written in .py files and later executed by the interpreter!