# Chapter 21: Esoteric: Generating parameters on the fly with @files¶

Note

Remember to look at the example code:

## Overview¶

The different Ruffus decorators connect up different tasks and generate Output (file names) from your Input in all sorts of different ways.

However, sometimes, none of them quite do exactly what you need. And it becomes necessary to generate your own Input and Output parameters on the fly.

Although this additional flexibility comes at the cost of a lot of extra inconvenient code, you can continue to leverage the rest of Ruffus functionality such as checking whether files are up to date or not.

## @files syntax¶

To generate parameters on the fly, use the @files with a generator function which yields one list / tuple of parameters per job.

For example:

from ruffus import *

# generator function
def generate_parameters_on_the_fly():
"""
returns one list of parameters per job
"""
parameters = [
['A.input', 'A.output', (1, 2)], # 1st job
['B.input', 'B.output', (3, 4)], # 2nd job
['C.input', 'C.output', (5, 6)], # 3rd job
]
for job_parameters in parameters:
yield job_parameters

# tell ruffus that parameters should be generated on the fly
@files(generate_parameters_on_the_fly)
sys.stderr.write("%d + %d => %d\n" % (extra[0] , extra[1], extra[0] + extra[1]))

pipeline_run()


Produces:

1 + 2 = 3 Job = [“A”, 1, 2] completed 3 + 4 = 7 Job = [“B”, 3, 4] completed 5 + 6 = 11 Job = [“C”, 5, 6] completed

Note

Be aware that the parameter generating function may be invoked more than once: * The first time to check if this part of the pipeline is up-to-date. * The second time when the pipeline task function is run.

The resulting custom inputs, outputs parameters per job are treated normally for the purposes of checking to see if jobs are up-to-date and need to be re-run.

## A Cartesian Product, all vs all example¶

The accompanying example provides a more realistic reason why you would want to generate parameters on the fly. It is a fun piece of code, which generates N x M combinations from two sets of files as the inputs of a pipeline stage.

The inputs / outputs filenames are generated as a pair of nested for-loops to produce the N (outside loop) x M (inside loop) combinations, with the appropriate parameters for each job yielded per iteration of the inner loop. The gist of this is:

#_________________________________________________________________________________________
#
#   Generator function
#
#        N x M jobs
#_________________________________________________________________________________________
def generate_simulation_params ():
"""
Custom function to generate
file names for gene/gwas simulation study
"""
for sim_file in get_simulation_files():
for (gene, gwas) in get_gene_gwas_file_pairs():
result_file = "%s.%s.results" % (gene, sim_file)
yield (gene, gwas, sim_file), result_file

@files(generate_simulation_params)
def gwas_simulation(input_files, output_file):
"..."

If get_gene_gwas_file_pairs() produces:
['a.sim', 'b.sim', 'c.sim']

and get_gene_gwas_file_pairs() produces:
[('1.gene', '1.gwas'), ('2.gene', '2.gwas')]


then we would end up with 3 x 2 = 6 jobs and the following equivalent function calls:

gwas_simulation(('1.gene', '1.gwas', 'a.sim'), "1.gene.a.sim.results")
gwas_simulation(('2.gene', '2.gwas', 'a.sim'), "2.gene.a.sim.results")
gwas_simulation(('1.gene', '1.gwas', 'b.sim'), "1.gene.b.sim.results")
gwas_simulation(('2.gene', '2.gwas', 'b.sim'), "2.gene.b.sim.results")
gwas_simulation(('1.gene', '1.gwas', 'c.sim'), "1.gene.c.sim.results")
gwas_simulation(('2.gene', '2.gwas', 'c.sim'), "2.gene.c.sim.results")


The accompanying code looks slightly more complicated because of some extra bookkeeping.

You can compare this approach with the alternative of using @product:

#_________________________________________________________________________________________
#
#        N x M jobs
#_________________________________________________________________________________________
@product(   os.path.join(simulation_data_dir, "*.simulation"),
formatter(),

os.path.join(gene_data_dir, "*.gene"),
formatter(),

# add gwas as an input: looks like *.gene but with a differnt extension