Python Metaprogramming: An example of how to use Abstract Syntax Trees (ast module) in Python to turn bare expressions (not bound to any target variable names) in an end-user configuration file into elements of a list.

model_config.py

"""
This file is made by the user, to set up model components for a Markov Chain 
Monte Carlo galaxy simulator.
For simplicity, I didn't want to force the user to bind everything to variables, 
or follow some particular naming scheme, or remember to add them to a list, etc. 
YAML or such seemed like overkill, since python keyword arguments already make it
pretty human-readable
"""
from psfMC.ModelComponents import Sky, PSF, Sersic
from psfMC.distributions import Normal, Uniform

# First component (modeling the brightness of the background sky)
Sky(adu=Normal(mu=0, tau=100))

# Point source component
PSF(xy=Uniform(lower=(60, 60), upper=(70, 70)),
    mag=Uniform(lower=18, upper=24))

# Sersic profile, modeling a galaxy under the point source
Sersic(xy=Uniform(lower=(60, 60), upper=(70, 70)),
       mag=Uniform(lower=22, upper=27.5),
       reff=Uniform(lower=1.0, upper=12.0),
       index=Uniform(lower=0.5, upper=8),
       axis_ratio=Uniform(lower=0.25, upper=1.0),
       angle=Uniform(lower=0, upper=360))

# A second sersic profile, modeling a fainter nearby galaxy
Sersic(xy=Uniform(lower=(42, 82), upper=(50, 90)),
       mag=Uniform(lower=22, upper=27.5),
       reff=Uniform(lower=1.0, upper=4.0),
       index=Uniform(lower=0.5, upper=8.0),
       axis_ratio=Uniform(lower=0.7, upper=1.0),
       angle=Uniform(lower=0, upper=360))

model_parser.py

from ast import *

_comps_name = 'components'


class ExprsToAssigns(NodeTransformer):
    """
    Walks the Abstract Syntax Tree from the parsed model file, and transforms
    bare expressions into augmented assignments that are tacked on to the end
    of the components list, ie:
    Sky(...)
    becomes:
    components += [Sky(...)]
    """
    def visit_Expr(self, node):
        return copy_location(AugAssign(
            target=Name(id=_comps_name, ctx=Store()),
            op=Add(),
            value=List(elts=[node.value], ctx=Load())
        ), node)


def component_list_from_file(filename):
    """
    Read in a file containing model components and return them as a list
    """
    with open(filename) as f:
        model_tree = parse(f.read())

    # Inject distribution and component imports. Put them at the beginning
    # so user imports may override them. level=1 means relative import, e.g.:
    # from .ModelComponents import *
    increment_lineno(model_tree, n=3)
    comps = ImportFrom(module='ModelComponents',
                       names=[alias(name='*', asname=None)], level=1)
    dists = ImportFrom(module='distributions',
                       names=[alias(name='*', asname=None)], level=1)
    model_tree.body.insert(0, comps)
    model_tree.body.insert(1, dists)

    # Insert a statement creating an empty list called components
    comp_list_node = Assign(targets=[Name(id=_comps_name, ctx=Store())],
                            value=List(elts=[], ctx=Load()))
    model_tree.body.insert(2, comp_list_node)

    # Transform bare components expressions into list append statements
    model_tree = ExprsToAssigns().visit(model_tree)
    fix_missing_locations(model_tree)

    exec(compile(model_tree, filename, mode='exec'))

    return locals()[_comps_name]