exists() of tool in use is never called

[bdbaddog]

This issue was originally created at: 2015-04-21 07:37:40.
This issue was reported by: knut3.
knut3 said at 2015-04-21 07:37:40

Hi,

I wrote a tool and put it into site_cons/site_tools/my_tool.py.
(http://scons.org/wiki/ToolsForFools)
According to the user guide "18.7. Where To Put Your Custom Builders and Tools"
there have to be two functions: exists() and generate().

I use my tool and it works fine thus generate() was called. But the exists()
function is never called. I put a "return False" there but no complains while
running scons...

knut

[nobodyinperson]

Wondering about this as well. Is one supposed to call exists() inside generate() manually?

The documentation really only mentions this:

A tool specification must include two functions: generate(env, **kw) and exists(env). The generate function modifies the environment referenced by env to set up variables so that the tool can be executed; it may use any keyword arguments that the user supplies in kw (see below) to vary its initialization. The exists function should return a true value if the tool is available.

What is the use of this exists() tool function, if it is not called upon Environment creation?

[mwichmann]

It's called when SCons initializes the tools, in a stanza that looks like this:

        for t in self.tools:
            tool = SCons.Tool.Tool(t)
            if tool.exists(env):
                env.Tool(tool)

It's not nearly as useful as it could be... if exists finds the program via a search, it knows where it is. But it can't return that path, because nobody is listening for that; so generate usually ends up searching for it again (or even, sigh, directly calling exists again). Some of the existing tools just return True from exists without even checking....

[bdbaddog]

@nobodyinperson - exists() is called by SCons when it loads the default tools for the platform. If you create your own tool, and then call env.Tool('mytool') exists() is not called.

See:

• https://github.com/SCons/scons/blob/master/src/engine/SCons/Tool/__init__.py#L1134
• https://github.com/SCons/scons/blob/master/src/engine/SCons/Tool/__init__.py#L1142
• https://github.com/SCons/scons/blob/master/src/engine/SCons/Tool/__init__.py#L1149

I'd say the current implementation should probably call exists() when env.Tool('mytool'), but as yet it doesn't. (Thus this bug)

So to sum up: Please do create a def exists() in your tool. env.Tool() does already reference it, but never calls it. When this bug is resolved your tool will fail..

[mwichmann]

This behavior got some doc updates in #3778, which does not resolve this issue.