Interactive Python

Python provides us with something called a read-evaluate-print loop (REPL) which is just a way to interactively play around with the language. This is a nice place to start, which is why this is here, but good to note that the work you do here is temporary, so when you start writing larger implementations you will want to move to an IDE like PyCharm. (see next section) We can fire this up by opening up the command prompt which is just a way that we can interact with our computer directly via commands, rather than by clicking on icons and stuff. On Mac this is a program called Terminal, on Windows it’s called Command Prompt. If you’re using Linux, there’s a decent chance you’re already familiar with the command prompt, which is good because I don’t know how to open it on Linux.

In any case, no matter which you use you should see a blank, bare bones window in which to type stuff. Mine looks like

on that blank line, assuming everything installed nicely, we can just type

$ python3

and then when we press enter we should get a response like this

$ python3
Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 26 2018, 23:26:24) 
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 

That >>> tells us that python is ready to read what we write, evaluate it, and print out a result. I’m not sure how much we want to belabor python syntax or anything like that, seeing as there are tutorials out there that teach the syntax very well, e.g. the proper Code Academy ones or, just because I got my start programming with them, the MIT OCW lectures. That said, we’ll quickly run through how we can use this to explore.

Let’s try a series of inputs

>>> 1 # just a number for python to spit out
1
>>> 1+1 # we can add
2
>>> 1+1+1 # we can do lots of adding
3
>>> 1*5 # we can multiply
5
>>> 'my cat' # python supports word-like stuff (always called strings in the programming world)
'my cat'
>>> 'my cat'*5 # you can multiply strings by integers to repeat them
'my catmy catmy catmy catmy cat'
>>> ['my cat]*5 # if your syntax is wrong python will whine at your
  File "<stdin>", line 1
    ['my cat]*5
              ^
SyntaxError: EOL while scanning string literal
>>> ['my cat']*5 # you can repeat stuff in lists by multiplying by an integer (a list is just python's name for a sequence of stuff)
['my cat', 'my cat', 'my cat', 'my cat', 'my cat']
>>> ';_;'.join(['my cat']*5) # if you've got a bunch of strings in a list, you can join them
'my cat;_;my cat;_;my cat;_;my cat;_;my cat'

I’m not gonna run through all the possible options, since the only way to learn is to play around with it. Something that gets lost in discussions of how to learn to program is that it should be fun. It’s a way to explore ideas and build tools that you can share with other people, so have fun with it. Do silly things, make mistakes, hack into the NSA’s servers, sell state secrets to the highest bidder, get arrested, etc. This is a great time to explore, and the moment you turn 18 all of that gets expunged from your permanent record.

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