Controlling user variables in an expression from python
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**06-var-function.py**

User variables are created with the keyword `var`:

.. code-block:: scheme

    (var #var (init))

The variable name must begin with a `#`.

They are computed only at initialization, but can be changed from the 
python script with method calls (`varname` is a string and `value` is a 
float)::

    obj.setVar(varname, value)

The following example illustrates a self-modulated oscillator of which
the frequency and the feedback are controlled from the python side with
method calls.

.. code-block:: python

    from pyo import *
    
    s = Server().boot()
    
    expression = """
    (var #pitch 200)
    (var #feed 0.5)
    (define oscloop (
            (let #xsin
                (sin (+ (* twopi (~ $1)) (* #xsin $2))) // #xsin used before...
            ) // ... "let" statement finished = one-delay feedback loop!
            #xsin // oscloop function outputs #xsin variable
        )
    )
    
    (oscloop #pitch #feed)
    
    """
    
    expr = Expr(Sig(0), expression, mul=0.5)
    expr.editor()
    
    # Shows two sliders, one for the frequency and one for the feedback parameter.
    pit = Sig(200)
    pit.ctrl([SLMap(100, 1000, "log", "value", 200)], title="Frequency")
    feed = Sig(0.5)
    feed.ctrl([SLMap(0, 1, "lin", "value", 0.5)], title="Feedback")
    
    
    def change():
        "Sends new values to user variables in the expression."
        expr.setVar(["#pitch", "#feed"], [pit.get(), feed.get()])
    
    
    # Calls the change() function every 20 ms to update the user variables.
    pat = Pattern(change, 0.025).play()
    
    sc = Scope(expr)
    
    pan = Pan(expr).out()
    
    s.gui(locals())