Attempting to access a name that hasn't been assigned will immediately raise a NameError. Therefore, "checking if a variable is defined" in Python is primarily about handling potential NameError exceptions or, more ideally, structuring your code to prevent them from occurring.

Let’s explore different methods to efficiently check if a variable exists in Python. Try-except block is Python’s preferred method for handling errors. Instead of checking if a variable exists beforehand, you try to access it and catch the NameError that occurs if the variable doesn't exist.

Advanced (Introspection): For special cases, you can check for the variable's name as a string in the output of locals() or globals(), but this should be avoided in normal application code. By defaulting to the None initialization pattern, you will write clearer, more predictable, and more maintainable Python code.

Each method comes with clear explanations and full code examples you can use immediately in your Python Keras projects. The most direct way to check if a variable is defined is by trying to access it inside a try block and catching a NameError if it’s not defined. This method is easy and works well in all Python environments. print(model)