WIP: raise ValueError if param.depends is used without arguments or without defining dependencies

[maximlt]

To raise an error in these cases where the depends decorator is used either without arguments or without providing any dependencies (same on methods on Parameterized classes).

@param.depends
def foo():
    pass

# ValueError: The depends decorator on the callable 'foo' cannot be used without arguments.

@param.depends()
def foo():
    pass

# ValueError: The depends decorator on the callable 'foo' require dependencies to be defined.

WIP because as such the following would raise an error (since internally Panel uses the depends decorator when it binds a callable) while the approach retained is to warn in this case (https://github.com/holoviz/panel/pull/3545).

import panel

def foo(val):
    return val

pn.panel(pn.bind(foo, val='bob'))

Emitting a warning instead of raising an error when no dependencies are defined may be a better idea (that Panel could silence).

[codecov-commenter]

Codecov Report

Merging #631 (5877097) into master (d960230) will increase coverage by 0.03%. The diff coverage is 88.37%.

@@            Coverage Diff             @@
##           master     #631      +/-   ##
==========================================
+ Coverage   82.91%   82.94%   +0.03%     
==========================================
  Files           5        5              
  Lines        3073     3079       +6     
==========================================
+ Hits         2548     2554       +6     
  Misses        525      525              

Impacted Files	Coverage Δ
param/parameterized.py	83.39% <88.37%> (+0.06%)	:arrow_up:

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[philippjfr]

I don't think this is correct, @param.depends() is documented and the behavior is well defined in the user guide:

Conversely, if you want to declare that a given method does not depend on any parameters at all, you can use @param.depends().

So I'm perfectly happy to disallow param.depends without calling it but we should not disallow param.depends().

[jbednar]

I'm not sure what the interaction with Panel is, but I agree with @philippjfr that depends() is not an error but depends is.

[maximlt]

I no longer see much value in the changes I made in this PR. As stated above@param.depends() is valid. @param.depends could either raise an error or be accepted and made equivalent to @param.depends(). As I don't want to decide that now and this is definitely super low priority, I'm just going to close this. Let's revisit when this comes up again, if ever!