Python File Input and Output Questions and Answers

Question 1

Which of the following is the most idiomatic and safest way to open a file for writing in Python, ensuring that it is always closed properly?

Accepted Answer

with open('data.txt', 'w') as file:
    file.write('content')

Answer

The `with` statement is the recommended way to handle file objects. It automatically takes care of closing the file when the block is exited, even if an exception occurs within the block. This prevents resource leaks and is more concise than a `try...finally` block.

Question 2

A developer needs to write a logging function that adds new entries to the end of a log file. If the log file does not exist, it should be created. The function also needs to be able to read from the file to check previous logs. Which file mode is the most appropriate for this scenario?

Accepted Answer

'a+'

Answer

The 'a+' mode is ideal for this use case. The 'a' stands for append, which means the file pointer is placed at the end of the file for writing, and if the file doesn't exist, it will be created. The '+' signifies that the file is opened for updating (both reading and writing). 'w+' would truncate the file, 'r+' would raise an error if the file doesn't exist, and 'a' would not allow reading.

Question 3

What is the primary difference in the return value between the `read()` and `readlines()` file methods in Python?

Accepted Answer

`read()` returns a single string containing the entire file content, while `readlines()` returns a list of strings, where each string is a line from the file.

Answer

The `file.read()` method, when called without an argument, reads the entire content of a file and returns it as a single string. In contrast, the `file.readlines()` method reads all lines from the file and returns them as a list of strings, with each string in the list representing one line and including the trailing newline character.

Question 4

After executing the following code, what will be the content of 'report.txt'?

```python
lines = ['- Item 1', '- Item 2', '- Item 3']
with open('report.txt', 'w') as f:
    f.writelines(lines)
```

Accepted Answer

- Item 1- Item 2- Item 3

Answer

The `writelines()` method takes an iterable of strings and writes each string to the file consecutively. However, it does not add any line separators like newlines (`
`) between the strings. Since the strings in the `lines` list do not contain newline characters, they will be concatenated directly in the file.

Question 5

Given a file named 'data.txt' containing the text 'ABCDEFG', what will the following Python code print?

```python
with open('data.txt', 'r') as f:
    f.read(3)
    f.seek(1)
    print(f.read(2))
```

Accepted Answer

BC

Answer

First, `f.read(3)` reads 'ABC' and moves the file pointer (cursor) to position 3 (after 'C'). Then, `f.seek(1)` moves the pointer to position 1 (after 'A'). Finally, `f.read(2)` reads the next two characters from the current position (1), which are 'B' and 'C'. The output is therefore 'BC'.

Python Practice Test

Python Practice Test

Python File Input and Output Questions and Answers