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For code you control, initialize the value or give it an explicit default; that is usually clearer than probing whether a name exists. If you deliberately need to inspect a namespace, test for the name as a string in the right mapping: "config_value" in globals() for the current module, or "config_value" in locals() for the current local namespace. The choice matters because Python resolves names according to scope, and an unresolved name raises NameError.
Choose the check that matches what you mean
“Does this variable exist?” can mean several different things: whether a name is bound in the module namespace, whether a local name is present, whether a value is not None, or whether an object has an attribute. Those are different questions, so there is no single universal existence check.
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| What you need to know | Use |
|---|---|
| Whether a name is in the current module namespace | "name" in globals() |
| Whether a name is in the current local namespace | "name" in locals() |
Whether a value is not None |
value is not None, after the name has been safely bound |
| Whether an object has an attribute | hasattr(obj, "attribute") |
The Python Language Reference explains that if a name is not found, Python raises NameError. A function can also raise UnboundLocalError, a subclass of NameError, when a local name is read before it has been assigned.
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When you control the value, initialize it instead
If your program determines whether a value will be available, set it deliberately before the conditional. This avoids making later code depend on probing Python’s namespaces.
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config_value = None
if config_value is not None:
use(config_value)
else:
use_default()This checks whether the value is None, not whether the variable name exists. If None is itself a valid value and must be distinguished from “not supplied,” use a unique sentinel:
MISSING = object()
config_value = MISSING
if config_value is MISSING:
use_default()
else:
use(config_value)The sentinel represents a separate “no value supplied” state, leaving None available as a real value.
Check a module-level name with globals()
globals() returns the current module namespace. Membership tests whether the string key is present there:
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use(config_value)
else:
use_default()This is appropriate when the question is specifically whether config_value is bound in the current module. It does not tell you whether a name exists only as a local variable in a function or in an enclosing function scope. See the Python Language Reference on execution and name resolution.
Use locals() only for the current local namespace
locals() exposes bindings in the current local namespace. Inside a function, you can ask whether a name appears in that namespace:
def process():
optional_value = 10
if "optional_value" in locals():
use(optional_value)Interpret this in the scope where the call occurs. It is not a general-purpose search for names in arbitrary enclosing scopes, and it is not a reliable way to create or assign a local variable. Python’s built-in documentation also notes that in optimized function scopes, locals() returns a fresh dictionary of current bindings. For namespace inspection, consult the documentation for locals().
Why a function assignment can cause UnboundLocalError
Python classifies a name as local throughout a function if that function assigns to it anywhere, unless the name is declared global or nonlocal. That means a name that appears to refer to a module-level value may instead be treated as a local name—and reading it before the assignment fails.
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def process():
print(value) # UnboundLocalError
value = "local value"Because process() assigns to value, Python treats it as local in the function body. The earlier read happens before that local binding. The fix is usually to restructure the function so it receives or initializes its value explicitly. If you truly intend to use the module binding, declare global value before using it; use nonlocal when referring to a binding in an enclosing function. The Python FAQ explains the local and global assignment rule.
Use hasattr() for attributes, not bare variable names
hasattr(obj, "attr") asks whether obj has an attribute named attr. It does not check whether a bare variable called attr exists.
if hasattr(settings, "timeout"):
use(settings.timeout)For a module attribute, the module is still an object, so hasattr(module, "name") can answer that attribute question. It is not a substitute for checking an unrelated local or global name. The built-in’s contract is documented under hasattr().
Avoid eval() for existence checks
Do not use eval("name") as a shortcut for checking whether a variable exists. It attempts to evaluate an expression rather than test membership in a namespace, and Python’s documentation warns that eval() can execute arbitrary code. Its access to nested scopes is also limited to nonlocal names already referenced in the calling scope. If you intentionally need to inspect a namespace, explicit membership in globals() or locals() makes the target scope clear. See the eval() documentation.
Quick Recap
Quick decision guide
- If your code owns the value’s lifecycle, initialize it or use an explicit default.
- If
Noneis a meaningful value, use a unique sentinel to represent absence. - If you specifically need to inspect module-level bindings, test the key in
globals(). - If you need to inspect the current local namespace, test the key in
locals()and keep its scope limitations in mind. - If you mean an object property, use
hasattr()with the object and attribute name.
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