How do I check if a variable exists in Python?

To check if a variable exists in Python, you can use several methods depending on the scope (global, local, or object attributes) and whether you want to handle exceptions explicitly. Here’s a detailed guide with examples:

1. Check for Global or Local Variables

Use the globals() or locals() dictionaries to check if a variable name exists in the current scope.

Example 1: Check Global Variables

x = 10  # Global variable

if 'x' in globals():
    print("Global variable 'x' exists.")
else:
    print("Global variable 'x' does NOT exist.")

# Output: Global variable 'x' exists.

Example 2: Check Local Variables

def my_function():
    y = 20  # Local variable
    if 'y' in locals():
        print("Local variable 'y' exists.")
    else:
        print("Local variable 'y' does NOT exist.")

my_function()  # Output: Local variable 'y' exists.

2. Check Object Attributes

Use hasattr() to check if an object (e.g., a class instance) has a specific attribute.

Example 3: Check Class/Instance Attributes

class MyClass:
    class_var = 30  # Class-level variable

    def __init__(self):
        self.instance_var = 40  # Instance-level variable

# Check class attribute
if hasattr(MyClass, 'class_var'):
    print("Class attribute 'class_var' exists.")  # Output: Exists

# Check instance attribute
obj = MyClass()
if hasattr(obj, 'instance_var'):
    print("Instance attribute 'instance_var' exists.")  # Output: Exists

3. Use try-except to Handle NameError

Attempt to access the variable and catch a NameError if it doesn’t exist.

Example 4: Check Any Variable (Global/Local)

try:
    print(z)  # Access a variable that may not exist
except NameError:
    print("Variable 'z' is not defined.")

# Output: Variable 'z' is not defined.

4. Check Variables in Nested Scopes

For nested functions or modules, use vars() or inspect the object’s __dict__.

Example 5: Nested Function Scope

def outer():
    outer_var = 50

    def inner():
        if 'outer_var' in globals():  # Check global scope
            print("outer_var is global.")
        elif 'outer_var' in locals():  # Check local scope of `inner`
            print("outer_var is local to inner.")
        else:
            print("outer_var not found.")

    inner()

outer()  # Output: outer_var not found (it's in `outer`, not `inner` or global)

5. Check for Deleted Variables

Even if a variable was deleted (del), globals()/locals() will reflect its absence.

Example 6: Variable Deletion

a = 100
del a  # Delete the variable

if 'a' in globals():
    print("'a' still exists.")
else:
    print("'a' no longer exists.")  # Output: 'a' no longer exists

Key Methods Summary

Method	Use Case
'var' in globals()	Check if a global variable exists.
'var' in locals()	Check if a local variable exists.
hasattr(obj, 'attr')	Check if an object attribute exists.
try-except NameError	General check for any variable existence.

Best Practices

1. Use globals()/locals() for checking variables in specific scopes.
2. Use hasattr() for object attributes or class-level variables.
3. Avoid try-except Overuse: Reserve for cases where variable existence is uncertain and errors need graceful handling.

By using these methods, you can safely check for variable existence in Python!