scons: How to deal with dynamic targets?

Question

I'm trying to automate my work of converting PDF to png file with scons. The tool used for my conversion is convert from ImageMagick.

Here's the raw command line:

1. convert input.pdf temp/temp.png
2. convert temp/*.png -append output.png

The first command will generate one PNG file for each page in PDF file, so the target of the first command is a dynamic file list.

Here's the SConstruct file I'm working on:

convert = Builder(action=[
    Delete("${TARGET.dir}"),
    Mkdir("${TARGET.dir}"),
    "convert $SOURCE $TARGET"])
combine = Builder(action="convert $SOURCE -append $TARGET")

env = Environment(BUILDERS={"Convert": convert, "Combine": combine})

pdf = env.PDF("input.tex")
pngs = env.Convert("temp/temp.png", pdf) # I don't know how to specify target in this line
png = env.Combine('output.png', pngs)
Default(png)

The code pngs = env.Convert("temp/temp.png", pdf) actually is wrong since the target is multiple files that I don't know how many before env.Convert is executed, so the final output.png only contains the first page of the PDF file.

Any hint is appreciated.

UPDATE:

I just found that I can use command convert input.pdf -append output.png to avoid the two-step conversion.

Still I'm curious how to handle the scenario when the intermediate temporary file list is unknown beforehand and requires a dynamic target list.

Answer 1

If you want to know how to do the original (convert and combine) situation you proposed, I would suggest creating a builder with a SCons Emitter. The emitter allows you to modify the list of source and target files. This works nicely for generated files that dont exist with a clean build.

As you mentioned, the convert step will generate multiple targets, the trick is you need to be able to "calculate" those targets in the emitter based on the source. For example, recently I created a wsdl2java builder and was able to do some simple wsdl parsing in the emitter to calculate all of the target java files to be generated (the source being the wsdl).

Here is a general idea of what the build scripts should look like:

def convert_emitter(source, target, env):
    # both and source and target will be a list of nodes
    # in this case, the target will be empty, and you need
    # to calculate all of the generated targets based on the
    # source pdf file. You will need to open the source file 
    # with standard python code. All of the targets will be
    # removed when cleaned (scons -c)
    target = [] # fill in accordingly
    return (target, source)

# Optionally, you could supply a function for the action
# which would have the same signature as the emitter
convert = env.Builder(emitter=convert_emitter,
                      action=[
                         Delete("temp"),
                         Mkdir("temp"),
                         "convert $SOURCE $TARGET"])
env.Append(BUILDERS={'Convert' : convert})

combine = env.Builder(action=convert_action, emitter=combine_emitter)
env.Append(BUILDERS={'Combine' : combine})

pdf = env.PDF('input.tex')
# You can omit the target in this call, as it will be filled-in by the emitter
pngs = env.Convert(source=pdf)
png = env.Combine(target='output.png', source=pngs)