Getting started

Python versions

You might have been somewhat overwhelmed by the versions of Python and PsychoPy:

  • Python2 to Python3 introduced various nice features (e.g. better string handling) but some incompatibilities (eg. print statement)

  • PsychoPy2 to PsychoPy3 introduced web experiments and Pavlovia.org sync tools

PsychoPy has been providing Python2 and Python3 versions for about a year, but stick to Python3 if possible.

The Coder Shell window

This is a great place just to try out a quick command and see what happens. You can check a little Python syntax and see the results of commands instantly.

Let’s type some commands into the shell panel and see what happens:

>>> a = 3
>>> b = a + 2
>>> b
5
>>> b == 5
True

Comments

NB Programmers can spend far more time reading code than writing it. Future-you will think of well-written comments as love letters from current-you.

In Python comments are usually indicated by the # symbol.

Short variable names need comments:
>>> # I hope I remember what this means later:
>>> a = 5 # participant age
Descriptive variable names become their own comments, and can be understood regardless of where they appear in the code:
>>> age = 5
>>> adjustedAge = age + 2

  • Remember, you’ll read these variable names more often than you type them, so take an extra second or two to make them descriptive, while saving on typing comments.

  • Use camelCase or under_scores as you prefer, but be consistent.

  • R programmers? No, you can’t use dots in variable names. They mean something in Python, analogous to the $ symbol in R.

Note

Annoyingly a British Mac keyboard doesn’t show you where the # is, but you can get it using Alt-3 if you’re running under OS X. If you’re using a British Mac keyboard under Windows you need Ctrl+Alt+3. Sigh!

Long comments

You can start/end a multi-line comment with three double-quotes:

"""This is a potentially long piece of text that
will be ignored. If it occurs at the start of a
function it becomes the help for that function
"""

In the PsychoPy Coder window you can comment out lines with Ctrl-' and undo that with Shift-Ctrl-' If you forget, it’s listed in the Edit menu

Shortcuts

In the shell window you can see available code-completion options by starting to type a command. Type these lines gradually, taking note of what happens when you type the ‘.’ in the second line:

>>> name = 'Jessica'
>>> name = name.upper()
>>> name
JESSICA

To repeat a previous command hit Alt-P on your keyboard. e.g. Repeat the name = 'Jessica' statement to go back to its original value.

Editing Scripts

Although the shell is a handy place to check a quick command, it’s often desirable to be able to repeat a set of commands without retyping them. Type this into the editor window and save the script somewhere (e.g. firstScript.py):

a = "hello"
print(a)
b = ' world'
a + b

Switch the bottom panel of the Coder view to show the Output from the script. Hit the Run button (or press Ctrl-R). You might have expected to see hello world but it didn’t appear. That’s because in running scripts nothing is printed to the output unless you explicitly request it. Change the last line to print(a+b).

Strings in Python can be defined using either ' or ". To actually include one of these marks within a string, enclose it in the other one, for example:

>>> text1 = "I don't like ham"
>>> text2 = 'She said "hello".'

If you don’t match your quotes in pairs, you’ll notice all of your code turn pink.


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