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The equivalent to optparse callback is basically to define __call__ in a subclass of Action. Pretty much the same amount of work because they both have complicated parameters.
The "callable" I (not fully confidently) proposed would just be a shorthand for defining __call__ in a subclass of Action when you don't care about any of the other command line info.
I guess, without further use cases for "callable" and given that you can subclass Action for other use cases, let's just do the action='write' version. Feel free to supply a patch, or I will when I get some time for argparse again.