PythonCrashCourse Pg.151. TypeError: cannot concatenate 'str' and 'NoneType' objects

Question

In my book , Python Crash Course : This code is given but it gives an error.

def make_pizza(*toppings):
    """Summarize the pizza we are about to make."""
    print("\nMaking a pizza with the following toppings:")
    for topping in toppings:
        print("- " + topping)

make_pizza(make_pizza("peperoni"))

make_pizza(make_pizza("mushroom",'green peppers','extra cheese'))

The traceback is as follows:

print("- " + topping)

TypeError: cannot concatenate 'str' and 'NoneType' objects

Q. What is the NoneType object here ? Is it topping? If yes, why?

Even when i use str() around topping , It gives me a funny output:

def make_pizza(*toppings):
"""Summarize the pizza we are about to make."""
print("\nMaking a pizza with the following toppings:")
for topping in toppings:
    print("- " + str(topping))

make_pizza(make_pizza("peperoni"))
make_pizza(make_pizza("mushroom",'green peppers','extra cheese'))

Output:

Making a pizza with the following toppings:

- peperoni

Making a pizza with the following toppings: 

- None

Making a pizza with the following toppings:
- mushroom
- green peppers
- extra cheese

Making a pizza with the following toppings:
- None

Q2. Why is displaying 2 outputs for each? - one with the list of toppings -and one with None ?

Answer 1

You shouldn't be passing the function to itself.

Replace these:

make_pizza(make_pizza("peperoni"))
make_pizza(make_pizza("mushroom",'green peppers','extra cheese'))

with

make_pizza("peperoni")
make_pizza("mushroom",'green peppers','extra cheese')

---

The reason that the error is happening is because you are passing the function make_pizza , which has no return value (it returns nothing), to itself.

However , the inner function still executes completely. That is, the second make_pizza in each of your examples is running properly, which is why you're getting two outputs.

For the second function though, you're effectively trying to run:

make_pizza(None)

Which of course leads to an error here:

print("- " + topping)

because topping is None .

---

The reason you sometimes saw - None in your output is because str(None) casts None to "None" (as in, a string containing the literal text None ).