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.

Practice

1. What happens when you open a file in Python with mode 'w' and write data to it?

easy

A. The file content is replaced with the new data.

B. The new data is added at the end of the file.

C. The file is opened for reading only.

D. The file content is duplicated.

Solution

1. Step 1: Understand file mode 'w'

   Opening a file with mode 'w' means write mode, which clears existing content.

2. Step 2: Effect of writing in 'w' mode

   Writing data in 'w' mode overwrites any existing content with the new data.

3. Final Answer:

   The file content is replaced with the new data. -> Option A

4. Quick Check:

   Overwrite = Replace content [OK]

Hint: Mode 'w' always replaces file content [OK]

Common Mistakes:

• Thinking 'w' appends data
• Confusing 'w' with 'a' mode
• Assuming 'w' opens file for reading

2. Which of the following is the correct way to open a file for appending text in Python?

easy

A. open('file.txt', 'w')

B. open('file.txt', 'r')

C. open('file.txt', 'a')

D. open('file.txt', 'x')

Solution

1. Step 1: Identify append mode

   Mode 'a' opens the file for appending, adding data at the end without deleting existing content.

2. Step 2: Check other modes

   'w' overwrites, 'r' reads only, 'x' creates new file and errors if exists.

3. Final Answer:

   open('file.txt', 'a') -> Option C

4. Quick Check:

   Append mode = 'a' [OK]

Hint: Use 'a' to add data without deleting old [OK]

Common Mistakes:

• Using 'w' when append is needed
• Confusing 'r' with append mode
• Using 'x' which fails if file exists

3. What is the output of this code?

lines = ['one', 'two']
with open('test.txt', 'w') as f:
    for line in lines:
        f.write(line + '\n')
with open('test.txt', 'a') as f:
    f.write('three\n')
with open('test.txt') as f:
    print(f.read())

medium

A. SyntaxError

B. three\n

C. one\ntwo\n

D. one\ntwo\nthree\n

Solution

1. Step 1: Write lines with 'w' mode

   The first block writes 'one' and 'two' each on new lines, overwriting any old content.

2. Step 2: Append 'three' with 'a' mode

   The second block adds 'three' on a new line at the end without removing previous lines.

3. Step 3: Read and print file content

   The last block reads all lines, so output is 'one\ntwo\nthree\n'.

4. Final Answer:

   one\ntwo\nthree\n -> Option D

5. Quick Check:

   Write 'w' then append 'a' = combined content [OK]

Hint: 'w' clears file, 'a' adds after existing content [OK]

Common Mistakes:

• Assuming 'a' overwrites content
• Forgetting newline characters
• Expecting only last write to appear

4. This code tries to add a line to a file but does not work as expected:

with open('data.txt', 'w') as f:
    f.write('Hello\n')
with open('data.txt', 'w') as f:
    f.write('World\n')

What is the problem?

medium

A. The second write overwrites the first line.

B. The file is opened in read mode instead of write mode.

C. The file is not closed before second write.

D. The write method cannot be used twice on the same file.

Solution

1. Step 1: Analyze first write

   The first block writes 'Hello' and creates or overwrites the file.

2. Step 2: Analyze second write with 'w'

   The second block opens the file again in 'w' mode, which clears previous content, then writes 'World'.

3. Final Answer:

   The second write overwrites the first line. -> Option A

4. Quick Check:

   Opening with 'w' overwrites content [OK]

Hint: Opening with 'w' erases old content [OK]

Common Mistakes:

• Thinking file appends automatically
• Believing file must be closed manually
• Assuming write() can only be called once

5. You want to keep a log of events in a file without losing old logs. Which code snippet correctly appends new logs without overwriting existing ones?

medium

A. with open('log.txt', 'w') as f:
f.write('New event\n')

B. with open('log.txt', 'a') as f:
f.write('New event\n')

C. with open('log.txt', 'r') as f:
f.write('New event\n')

D. with open('log.txt', 'x') as f:
f.write('New event\n')

Solution

1. Step 1: Understand log file needs

   Logs should be added without deleting old entries, so appending is needed.

2. Step 2: Choose correct mode

   Mode 'a' appends data; 'w' overwrites, 'r' is read-only, 'x' creates new file and errors if exists.

3. Final Answer:

   with open('log.txt', 'a') as f: f.write('New event\n') -> Option B

4. Quick Check:

   Append mode 'a' preserves old data [OK]

Hint: Use 'a' mode to add without deleting old logs [OK]

Common Mistakes:

• Using 'w' and losing old logs
• Trying to write in 'r' mode
• Using 'x' which fails if file exists