Python: Convert string into function name; getattr or equal?

Question

I am editing PROSS.py to work with .cif files for protein structures. Inside the existing PROSS.py, there is the following functions (I believe that's the correct name if it's not associated with any class?), just existing within the .py file:

...
def unpack_pdb_line(line, ATOF=_atof, ATOI=_atoi, STRIP=string.strip):
...
...
def read_pdb(f, as_protein=0, as_rna=0, as_dna=0, all_models=0,
    unpack=unpack_pdb_line, atom_build=atom_build):

I am adding an optons parser for command line arguments, and one of the options is to specify an alternate method to use besides unpack_pdb_line. So the pertinent part of the options parser is:

...
parser.add_option("--un", dest="unpack_method", default="unpack_pdb_line", type="string", help="Unpack method to use. Default is unpack_pdb_line.")
...
unpack=options.unpack_method

However, options.unpack_method is a string and I need to use the function with the same name as the string inside options.unpack_method. How do I use getattr etc to convert the string into the actual function name?

Thanks,

Paul

Answer 1

Usually you just use a dict and store (func_name, function) pairs:

unpack_options = { 'unpack_pdb_line' : unpack_pdb_line,
                   'some_other' : some_other_function }

unpack_function = unpack_options[options.unpack_method]

Answer 2

If you want to exploit the dictionaries (&c) that Python's already keeping on your behalf, I'd suggest:

def str2fun(astr):
  module, _, function = astr.rpartition('.')
  if module:
    __import__(module)
    mod = sys.modules[module]
  else:
    mod = sys.modules['__main__']  # or whatever's the "default module"
  return getattr(mod, function)

You'll probably want to check the function's signature (and catch exceptions to provide nicer error messages) e.g. via inspect, but this is a useful general-purpose function. It's easy to add a dictionary of shortcuts, as a fallback, if some known functions full string names (including module/package qualifications) are unwieldy to express this way.

Note we don't use __import__'s result (it doesn't work right when the function is in a module inside some package, as __import__ returns the top-level name of the package... just accessing sys.modules after the import is more practical).