Default Parameters & Variable Scope
Sometimes you want a function parameter to be optional. You can give it a default fallback value inside the definition.
Default Parameter Values
If whoever calls the function does not provide an argument, Python automatically uses the default value:
def greet(name="Guest"):
print(f"Welcome, {name}!")
greet() # Uses default: Welcome, Guest!
greet("Alice") # Uses argument: Welcome, Alice!
:::danger Never Use Lists as Default Parameters
When creating optional arguments, never put an empty list [] or dictionary {} as a default parameter! Python creates default parameters only once when reading the program. If you modify that list, every subsequent function call shares the exact same modified list:
# Wrong - shares the same list across all calls
def add_item(item, box=[]):
box.append(item)
return box
# Right - use None as a placeholder
def add_item_safe(item, box=None):
if box is None:
box = []
box.append(item)
return box
:::
Local vs. Global Variables (Scope)
Scope determines where variables can be seen and used in your program.
1. Local Variables (Private to the Function)
Any variable created inside a function only exists inside that function. As soon as the function finishes running, Python destroys the local variable:
def calculate():
secret = "hidden_code"
# print(secret) # NameError: name 'secret' is not defined outside the function!
2. Global Variables (Visible Everywhere)
Variables created outside at the top level of your script are global. Functions inside the script can read them:
APP_NAME = "Python Lab"
def show_banner():
print(f"Running on {APP_NAME}")
Common Beginner Mistakes
Trying to read a local variable from outside the function
# Wrong - total_score only exists inside calculate()
def calculate():
total_score = 100
calculate()
print(total_score) # NameError!
# Right - return the value so outside code can save it
def calculate():
return 100
total_score = calculate()
print(total_score)
What's Next?
Let's explore how to write functions that accept any number of inputs using *args and **kwargs.