Update objects when path changes for MultiFileSelector and FileSelector

[maximlt]

Fixes #545 (only addresses the update path issue, not the default value)

import param

class A(param.Parameterized):
    fs = param.FileSelector(path='/usr/share/*')
    mfs = param.MultiFileSelector(path='/usr/share/*')

a = A()

print(a.fs is None)  # True
paths = a.param.fs.objects.copy()
a.param.fs.path = '/usr/bin/*'
print(a.param.fs.objects == paths)  # False

print(a.mfs is None)  # True
paths = a.param.mfs.objects.copy()
a.param.mfs.path = '/usr/bin/*'
print(a.param.mfs.objects == paths)  # False

[jbednar]

The path support looks great now, thanks! I'm confused about the default handling, though. #545 says that the default is None here and in Multifileselector (below), which appears to be the case after these diffs, but it also says that the default value handling isn't changed, yet here and below the diffs are explicitly changing the default value handling. What am I missing?

[maximlt]

Before these diffs default is None indeed, despite the fact that it seems to be set to the first path in update(). This is because the base class constructor (Selector) is called with default=default (note that it's not default=self.default) and empty_default=True which means that the autodefault mechanism isn't used.

I was also slightly confused by how default should behave. When the objects attribute of a Selector is changed the default value isn't updated:

import param

class P(param.Parameterized):
    s = param.Selector(objects=list('abc'))

p = P()

print(p.param.s.default)  # 'a'

p.param.s.objects = list('def')

print(p.param.s.default)  # 'a'

While FileSelector was not setting default when initialized, it was however possible to update it by calling update(), as long as objects.

class A(param.Parameterized):
    fs = param.FileSelector(path='/usr/share/*')

a = A()

print(a.param.fs.default)  # None

a.param.fs.path = '/usr/bin/*'
a.param.fs.update()

print(a.param.fs.default)  # '/usr/bin/2to3-'

With this PR, default is None if not declared by the user. If declared and update() is called, it is set to None if the previous default path is not one of the new available paths listed in objects:

class A(param.Parameterized):
    fs1 = param.FileSelector(path='/usr/share/*')
    fs2 = param.FileSelector(path='/usr/share/*', default='/usr/share/CSI')

a = A()

print(a.param.fs1.default, a.param.fs2.default)  # None, '/usr/share/CSI'

a.param.fs1.path = '/usr/share/*'
a.param.fs2.path = '/usr/share/*'
a.param.fs1.update()
a.param.fs2.update()

print(a.param.fs1.default, a.param.fs2.default)  # None, '/usr/share/CSI'

a.param.fs1.path = '/usr/bin/*'
a.param.fs2.path = '/usr/bin/*'
a.param.fs1.update()
a.param.fs2.update()

print(a.param.fs1.default, a.param.fs2.default)  # None, None

But this is quite an arbitrary decision, default could also be not reset and just stick to whatever the first value is declared when the Parameter is initialized (as done by Selector). Let me know what you think :)

(update() needs a docstring, so once we settle on a behavior it can be explicitly described)

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[maximlt]

@jbednar I've:

• reverted to the original behavior for default, which is (for both FileSelector and MultiFileSelector) to have default=None if not specified by the user, and that can then be updated by calling '.update()', to 'self.objects[0]' for FileSelector and 'self.objects' for MultiFileSelector
• Removed _on_set (a method called when an attribute is set on a Parameter) since it was just not working in this case (it was calling .update(), itself using self.path which still pointed to the old attribute value)
• Updated the user guide to describe these behaviors

[philippjfr]

This looks good. @maximlt can you advise if this PR still should be merged and @jbednar can you please either approve and merge or suggest what other fixes are needed?

[jbednar]

Seems like an appropriate change to me. We might want to revisit after #605 to make the default always be the first value or all values (for multi) unless the user has explicitly provided a default of None (currently not possible to detect).

[jbednar]

This appears to say two different values for the default, which presumably can't be true?

[jbednar]

I think is is ready for revision by @maximlt now that #605 was merged. I think it makes more sense for the default always be the first value or all values (for multi) unless the user has explicitly provided a default of None; None is not a very useful default. In any case, the PR title needs updating to reflect the actual changes in the diffs.