Is it bad practice to use python's getattr extensively?

Question

I\'m creating a shell-like environment. My original method of handleing user input was to use a dictionary mapping commands (strings) to methods of various classes, making u

Answer 1

The difference between direct attribute access and using getattr() should be fairly negligible. You can tell the difference between the two versions' bytecodes by using Python's dis module to compare the two approaches:

>>> import dis
>>> dis.dis(lambda x: x.foo)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_ATTR                0 (foo)
              6 RETURN_VALUE        
>>> dis.dis(lambda x: getattr(x, 'foo'))
  1           0 LOAD_GLOBAL              0 (getattr)
              3 LOAD_FAST                0 (x)
              6 LOAD_CONST               0 ('foo')
              9 CALL_FUNCTION            2
             12 RETURN_VALUE  

It does, however, sound like you are developing a shell that is very similar to how the Python library cmd does command line shells. cmd lets you create shells that executes commands by matching the command name to a function defined on a cmd.Cmd object like so:

import cmd

class EchoCmd(cmd.Cmd):
    """Simple command processor example."""

    def do_echo(self, line):
        print line

    def do_EOF(self, line):
        return True

if __name__ == '__main__':
    EchoCmd().cmdloop()

You can read more about the module at either the documentation, or at http://www.doughellmann.com/PyMOTW/cmd/index.html

Answer 2

does getattr have the same problems as eval?

No -- code using eval() is terribly annoying to maintain, and can have serious security problems. Calling getattr(x, "foo") is just another way to write x.foo.

will I be taking a hit to the efficiency of my shell

It will be imperceptibly slower if the command isn't found, but not enough to matter. You'd only notice it if doing benchmarks, with tens of thousands of entries.