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To check whether a bare name is defined in Python, first decide whether you need to inspect a namespace or whether your code should initialize the value. For a module-level name, test membership in globals(); for an explicitly managed value, initialize it or use a default. A variable holding None is still defined, so name existence and value checks are different questions.
Choose the check that matches what you mean
- Is this name in the current module namespace? Use
"name" in globals(). - Is this name among the current local bindings? Use
"name" in locals(), with the scope caveats below. - Does this object have an attribute? Use
hasattr(obj, "attribute"). - Does my program have a value to use? Initialize it, or use a default or sentinel.
Python resolves names according to the environment of the current code block. If it cannot find a name, it raises NameError, as described in the Python Language Reference, “Execution model”.
Check a module-level name with globals()
globals() returns the current module’s global namespace. Membership tests whether a key is present there; it does not test every name Python might resolve, nor does it inspect a function’s local or enclosing bindings.
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use(config_value)
else:
use_default()This pattern is useful when the intended question is specifically whether the module namespace contains that key. It is not a general-purpose way to ask whether a name exists anywhere in the current scope.
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Use locals() only for the current local namespace
locals() exposes bindings in the current local namespace, but its behavior depends on the scope. In optimized function scopes, it returns a fresh dictionary of current bindings. A membership check can inspect what is currently present:
def process():
value = 12
if "value" in locals():
use(value)Do not treat this as a universal inspection method for enclosing scopes, or as a way to create or assign a local variable by changing the returned mapping. For name-resolution rules and namespace details, see the Python built-ins documentation for locals().
Prefer initialization when your code controls the value
If your code determines whether a value will be supplied, make that state explicit rather than probing for an undeclared name:
config_value = None
if config_value is not None:
use(config_value)
else:
use_default()This tests whether the value is None, not whether the name exists. If None is a valid value that must be distinguished from “not provided,” use a unique sentinel:
MISSING = object()
config_value = MISSING
if config_value is MISSING:
use_default()
else:
use(config_value)The sentinel makes absence distinct from any ordinary value, including None. This is a design pattern for values your code manages, not a universal replacement for checking a namespace.
Understand why a function can raise UnboundLocalError
A function-local assignment affects how Python classifies that name throughout the function body. If a function assigns to a name anywhere in its body, Python treats it as local unless the function declares it global or nonlocal. Reading it before its local binding can raise UnboundLocalError, a subclass of NameError.
value = "module value"
def process():
print(value) # Raises UnboundLocalError
value = "local value"The assignment makes value local throughout process(); the earlier read does not fall back to the module value. Check the scope and move or redesign the assignment rather than trying to solve this with a global namespace membership test. The Python FAQ’s local and global variable rules explains this classification.
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hasattr(obj, "attr") asks whether an object has an attribute named attr. It does not check whether a bare variable name is defined:
if hasattr(settings, "theme"):
use(settings.theme)This is appropriate when settings.theme is the value you intend to inspect. Likewise, a module can be treated as an object when you specifically want to test for one of its attributes. The documented contract is about an object and an attribute-name string; see Python’s hasattr() documentation.
Avoid eval() for checking names
eval("name") is not a safer or clearer existence check. It evaluates code, and its access to nested scopes is limited to nonlocals already referenced in the calling scope. If you need to inspect a namespace, use membership in the relevant mapping instead. See the Python documentation for eval() for its behavior and warning.
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