Overwrite vs append behavior in Python - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when overwriting a dictionary key?

Consider the following Python code that assigns values to the same dictionary key multiple times. What will be the final content of the dictionary?

Python

d = {}
d['key'] = 1
d['key'] = 2
d['key'] = 3
print(d)

A {'key': 3}

B {'key': 1}

C {'key': [1, 2, 3]}

D KeyError

💡 Hint

Think about what happens when you assign a new value to an existing key in a dictionary.

Assigning a value to an existing key in a dictionary overwrites the old value. So the final value for 'key' is 3.

❓ Predict Output

intermediate

What happens when appending to a list inside a dictionary?

Look at this code that appends values to a list stored in a dictionary. What is the output?

Python

d = {'numbers': []}
d['numbers'].append(1)
d['numbers'].append(2)
d['numbers'].append(3)
print(d)

A TypeError

B {'numbers': 3}

C {'numbers': []}

D {'numbers': [1, 2, 3]}

💡 Hint

Appending adds items to the existing list inside the dictionary.

The list under 'numbers' grows with each append call, so the final list is [1, 2, 3].

❓ Predict Output

advanced

What is the output of this code mixing overwrite and append?

What will be printed by this code?

Python

d = {}
d['a'] = [1]
d['a'].append(2)
d['a'] = [3]
d['a'].append(4)
print(d)

A {'a': [3, 4]}

B {'a': [1, 2, 3, 4]}

C {'a': [1, 2]}

D AttributeError

💡 Hint

Notice when the list is overwritten and when append is called.

After overwriting 'a' with [3], the previous list is lost. Then 4 is appended to the new list, so 'a' is [3, 4].

❓ Predict Output

advanced

What error occurs when appending to a non-list value?

What happens when you try to append to a dictionary value that is an integer?

Python

d = {'x': 10}
d['x'].append(5)

A KeyError

B TypeError

C AttributeError

D No error, output: {'x': [10, 5]}

💡 Hint

Think about what types support the append method.

Integers do not have an append method, so calling append on d['x'] raises AttributeError.

🧠 Conceptual

expert

How many items are in the dictionary after this code runs?

Consider this code snippet. How many key-value pairs does the dictionary contain at the end?

Python

d = {}
for i in range(3):
    d['key'] = i
print(len(d))

A 3

B 1

C 0

D KeyError

💡 Hint

Think about whether the key 'key' is added multiple times or overwritten.

The key 'key' is overwritten in each loop iteration, so only one key remains in the dictionary.