The Tool Desk
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Use a dictionary for a fixed set of commands
A dispatch dictionary is an explicit allowlist: each accepted command points to the function that should handle it. The string can be a simple external command such as "go", even if the implementation function is named start. This is the approach recommended by the Python Software Foundation’s Programming FAQ.
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def start():
return "started"
def stop():
return "stopped"
actions = {"go": start, "halt": stop}
user_command = "go"
result = actions[user_command]()The dictionary stores function objects, not function-name strings. Indexing it resolves the command to a callable; the final parentheses invoke that callable. If input may contain an unsupported command, handle the missing key explicitly:
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...
def delete_user():
...
commands = {"add": add_user, "delete": delete_user}
try:
function = commands[input_value]
except KeyError:
raise ValueError(f"Unknown command: {input_value!r}")
result = function()Use getattr for an attribute on a known object
If the requested function is a method or attribute of an object, class, or module you already have, getattr can resolve it by name. Supplying a default, such as None, avoids an AttributeError when the attribute is absent. Check that the result is callable before invoking it:
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method = getattr(service, method_name, None)
if not callable(method):
raise ValueError("Unknown method")
result = method()When a name comes from an external caller, checking that it is callable is not authorization: an object may expose callable attributes that the caller should not be allowed to invoke. Restrict permitted names with an allowlist or use a dispatch dictionary whose entries represent the approved operations.
Import a module separately when its name is dynamic
If a string identifies a module, load that module with importlib.import_module, then resolve the function attribute. Importing a module and looking up a function on it are separate steps:
import importlib
module = importlib.import_module("my_package.handlers")
handler = getattr(module, "handle_request")
result = handler()Only import modules your application expects and trusts. If module or function names are provided externally, validate them against an allowlist appropriate to the application.
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locals() and globals() are scope lookups
These functions expose namespaces, but they are not general-purpose dispatch registries. Which names are present depends on scope, and the behavior of local namespaces has version-specific details; Python’s documentation notes changed locals() semantics in Python 3.13. When the accepted command set is known, a dedicated dictionary makes the permitted operations clearer and avoids relying on incidental names in a scope.
eval() and exec() execute code
eval evaluates a Python expression; exec executes a suite of statements. Neither is needed to look up a function that already exists. The Python 3.14.8 built-in functions reference warns that calling eval with untrusted user-supplied input leads to security vulnerabilities, and gives a corresponding warning for exec. Restricting __builtins__ is not a security mechanism.
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