[MacPorts] #59839: Record default python and perl versions

[MacPorts]

#59839: Record default python and perl versions
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  Reporter:  mf2k         |      Owner:  (none)
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by ryandesign):

 Support the "default python version" is 3.7, and that a hypothetical port
 declares dependencies based on that, and 3.7-specific paths get baked into
 the files the port installs. Let us suppose a user has installed this
 port.

 Then suppose the "default python version" is changed to 3.8. This would
 change the dependencies and files installed by this hypothetical port.
 Logically, we would want the port to be rebuilt, but we don't have a
 mechanism for that to occur. The only mechanisms by which a port would be
 rebuilt are if the version/revision/epoch changes, the OS major version
 changes, or the default c++ standard library changes. We can't really
 implement your suggestion without this.

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Ticket URL: <https://trac.macports.org/ticket/59839#comment:1>
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