What use is of this 'cast=float ,'?

[Tim Chase]

[rearranging for easier responding]

On 2017-10-27 13:35, Robert wrote:
> self.freqslider=forms.slider(
>      parent=self.GetWin( ),
>      sizer=freqsizer,
>      value=self.freq,
>      callback= self.setfreq,
>      minimum=−samprate/2,
>      maximum=samprate/2,
>      num_steps=100,
>      style=wx.SL_HORIZONTAL,
>      cast=float ,
>      proportion=1,
> )
> I am interested in the second of the last line.
> 
>      cast=float ,

The space doesn't do anything.  You have a parameter list, so the
comma just separates "cast=float" from the next parameter,
"proportion=1". The "cast=float" passes the "float()" function as a
way of casting the data.  In this case, it likely expects a function
that takes a number or string, and returns a number that can be used
to render the slider's value/position.

You could create one that works backwards:

  def backwards_float(input):
    return -float(input) # note the "-" inverting the interpreted
  value

  forms.slider(
    …
    cast=backwards_float,
    …
    )

> I've tried it in Python. Even simply with 
> 
> float

This is just the float() function.

> it has no error, but what use is it?

It's good for passing to something like the above that wants a
function to call.  The body of the function likely has something like

   resulting_value = cast(value)

which, in this case is the same as

   resulting_value = float(value)

> I do see a space before the comma ','. Is it a typo or not?

I think it's unintended.


The whole question started off peculiar because outside of a
function-invocation

   thing = other,

with the comma creates a one-element tuple and assigns the resulting
tuple to "thing"

  >>> x = 42
  >>> x
  42
  >>> y = 42,
  >>> y
  (42,)

Usually people will be more explicit because that comma is easy to
miss, so they'll write

  >>> z = (42,)
  >>> z
  (42,)

so that later people reading the code know that it's intended to be a
one-element tuple.

-tkc