[Scons-users] Value/Alias Nodes default to up-to-date

Bill Deegan bill at baddogconsulting.com
Sun Jan 2 23:16:32 EST 2022 

Value can be the target of a builder.
Additionally you might use Value() as a source were Value's value is
generated (think time stamp, git revision, other string type info (though
it doesn't have to be only Value)).
Basically anything that's not a file but you might want to use it to decide
if your build is out of date and/or to build command line for any Action()..

Note there are some outstanding (already filed on github) issues around
supporting Value() as the target and it's out of dated'ness, and also
supporting scanners reading Value() nodes.

Please feel free to pop on the discord server and ask all types of general
questions in your exploration of SCons.

We always like to help new users learning SCons.
There are notable differences between SCons and other build systems which
can lead some users to use logic which "worked" on other build systems (but
was never really correct) which doesn't work and is very painful to hack
together with SCons (Because you don't specify order with SCons, only
dependencies..)



On Sun, Jan 2, 2022 at 8:06 PM Brad Kraemer <computerpro_58 at hotmail.com>
wrote:

> I can't actually give a more concrete example. I'm just testing different
> aspects of SCons. i.e. reading through the man-page (and user guide), Oh
> that looks interesting...wonder how that works...Ah I see, that could be
> useful when doing X/Y/Z (rinse/repeat)
>
>
> In this particular example, I came across the note that said Alias's were
> in a separate namespace. So I thought I'd explore that a bit. And then I
> came across Value's which also do there own thing. I just thought it odd
> that they were considered up-to-date right on creation, although your
> explanation equally makes sense (created as up-to-date, so why should they
> be built, unless you explicitly request they AlwaysBuild). So thank-you for
> clearing that up (all is is working as designed).
>
>
> One last thing. Could you give me a more concrete example of when a
> Value() would be used? Alias() is fairly obvious, but the manual example
> for Value() is a bit convoluted.
>
>
> On 2022-01-02 10:52 p.m., Bill Deegan wrote:
>
> Can you give a more concrete example of what you're trying to do?
> Sounds like you want a target which isn't built unless explicitly
> requested?
> Is that correct?
> An your current approach is to use an Action for that?
>
> Lots of users dig around the docs and come up with some poorly constructed
> overcomplicated "solution".
> I'm trying to help you avoid doing such.
>
> Yes. If any node has no children or the children are up to date, then it
> will not rebuild the node.. (as you'd expect).
>
>
> On Sun, Jan 2, 2022 at 5:37 PM Brad Kraemer <computerpro_58 at hotmail.com>
> wrote:
>
>> Simply that I can use the Node returned by Alias() in places where a Node
>> is expected (for instance in the example I've given Depends(t1, a1)).
>>
>> Now you may ask why wouldn't I use Command(), since that it's defined
>> purpose? In this case I wanted the special property of Alias Node's (they
>> exist in their own namespace, outside the standard FS namespace). Well why
>> would I need this target to exist outside the standard FS namespace? Simply
>> because I have some internal Action's that I want to run (in the process of
>> building) that are not specific to any target (that the user can specify).
>> i.e. $ scons / , or $ scons . is not going to pick up these Alias Node
>> targets (and thats exactly what I want).
>>
>>
>> My reasons for wanting this are more abstract at this point (as I said
>> I'm experimenting around the limitations of SCons). But to save some time
>> here, I'm literally looking for a Yes or No answer, with regards to the
>> Alias/Value node's up-to-date behavior.
>>
>> Specifically, check the file: /SCons/Node/__init__.py:1520
>>
>>  def children_are_up_to_date(self):
>>      """Alternate check for whether the Node is current:  If all of
>>      our children were up-to-date, then this Node was up-to-date, too.
>>      The SCons.Node.Alias and SCons.Node.Python.Value subclasses
>>      rebind their current() method to this method."""
>>      # Allow the children to calculate their signatures..
>>      self.binfo = self.get_binfo()
>>      if self.always_build:
>>          return None
>>      state = 0
>>      for kid in self.children(None):
>>          s = kid.get_state()
>>          if s and (not state or s > state):
>>              state = s
>>      return (state == 0 or state == SCons.Node.up_to_date)
>>
>>
>> *Both Value/Alias Node's use this method to cite there up-to-date state.
>> As written, with either no children or children with unchanging states a
>> Value/Alias will always be considered up-to-date (i.e. it is considered
>> built/up-to-date directly after creation). Is this correct?
>>
>>
>> If this behavior is correct what is the purpose of the 'built_value'
>> business on Value Node's? I don't fully grasp how Value Node's are meant to
>> work in the standard case...
>>
>>
>> On 2022-01-02 8:05 p.m., Bill Deegan wrote:
>>
>> Still don't really understand what you're trying to do..
>>
>> What do you mean by this "allowing me to make Action()'s with dependency
>> orderings"?
>>
>> On Sun, Jan 2, 2022 at 3:10 PM Brad Kraemer <computerpro_58 at hotmail.com>
>> wrote:
>>
>>> * Fixed subject line (the original line from the mailer daemon added
>>> JUNK, as this one doesn't allow unconfirmed posters, i.e. I should have
>>> waited for the mailing-list confirmation before sending the first e-mail
>>> off)
>>>
>>>
>>> Currently, just experimenting with some SCons features. At this time I
>>> was creating an Alias() because I wanted the Alias() object (which is
>>> namespaced outside the regular FS namespace), allowing me to make
>>> Action()'s with dependency orderings. While not having these actions kicked
>>> off by running something like '$ scons /' (which something like Command()
>>> would).
>>>
>>>
>>> I realize this is outside the intended use of these functions. However,
>>> the doc's don't forbid it so it's clean in my eyes.
>>>
>>> The Command() part is just for something to depend on (can replace it
>>> with Program() or any other builder, my issue is with the Alias() lines).
>>>
>>>
>>> Your reasoning makes sense, however, if my Alias is never out of date
>>> (i.e. on creation it's meant to be considered already built), why can I run
>>> AlwaysBuild() and thus force it to build (is this the equivalent of
>>> rebuilding something already up-to-date)?
>>>
>>> Just looking at the sources, it seems these Node's are meant to be built
>>> at least once (however, Value() in particular does seem to have a parameter
>>> to be considered built on creation). Or am I off base with this?
>>>
>>>
>>> On 2022-01-02 5:30 p.m., Bill Deegan wrote:
>>>
>>> Maybe you explain what you're trying to do, rather than how you're
>>> currently trying to do it?
>>> Because this looks like a mess of hard to understand code and there's
>>> very likely a better way to do this.
>>>
>>> Those aliases would never be out of date with a static source.
>>> Your command has no source listed, so it will also never be out of
>>> date..(after the first time, if the file t1 was actually built).
>>>
>>> -Bill
>>>
>>>
>>> On Sun, Jan 2, 2022 at 2:20 PM Bill Deegan <bill at baddogconsulting.com>
>>> wrote:
>>>
>>>> also why **JUNK** in subject?
>>>>
>>>> On Sun, Jan 2, 2022 at 2:20 PM Bill Deegan <bill at baddogconsulting.com>
>>>> wrote:
>>>>
>>>>> What output do you see when you run your example?
>>>>>
>>>>>
>>>>> On Sun, Jan 2, 2022 at 7:56 AM Brad Kraemer <
>>>>> computerpro_58 at hotmail.com> wrote:
>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> I filed a GitHub Issue <https://github.com/SCons/scons/issues/4079>
>>>>>> and got bounced to this mailing list. So here we go again:
>>>>>>
>>>>>> Value/Alias nodes (as created by the Alias() and Value()
>>>>>> functions/methods receptively) default to up-to-date.
>>>>>>
>>>>>> SCons 4.1.0
>>>>>>
>>>>>> Python 3.9
>>>>>>
>>>>>> Installed from nixpkgs (NixOS, $ nix-env -iA nixos.scons)
>>>>>>
>>>>>> Linux
>>>>>>
>>>>>> Reproduction (SConstruct):
>>>>>>
>>>>>> def print_target_1(target, source, env):
>>>>>>    print("Target 1 built")
>>>>>>
>>>>>> def print_alias_1(target, source, env):
>>>>>>    print("[I'm not getting called]")
>>>>>>
>>>>>> def print_alias_2(target, source, env):
>>>>>>    print("I'm getting called because of AlwaysBuild() workaround")
>>>>>>
>>>>>> t1 = Command("t1", [], Action(print_target_1, cmdstr=None))
>>>>>> Default(t1)
>>>>>>
>>>>>> v = Value("_")
>>>>>> a1 = Alias("a1", v, Action(print_alias_1, cmdstr=None))
>>>>>> a2 = Alias("a2", v, Action(print_alias_2, cmdstr=None))
>>>>>> Depends(t1, a1)
>>>>>> Depends(t1, a2)
>>>>>>
>>>>>> AlwaysBuild(a2)
>>>>>>
>>>>>>
>>>>>> Invoke via executing '$ scons' from the directory where the above
>>>>>> SConstruct file exists.
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> simplejack-src
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> Scons-users mailing list
>>>>>> Scons-users at scons.org
>>>>>> https://pairlist4.pair.net/mailman/listinfo/scons-users
>>>>>>
>>>>>
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