07 Files

Question 1

What are the differences between the OSs Unix and Windows, when it comes to directories/hierarchies?

Answer 1

Unix:
- The first directory is called “root directory”.
- The character “/” is used to separate directories in a path.

Windows:
- The first directory is your hard drive, e.g. C:
- The character Blackslash “" is used to separate directories in a path.

Question 2

How is the current folder called?

Answer 2

working directory

Question 3

What are the absolute and relative path of a file?

Answer 3

Absolute path:
- includes the root directory
- is independent of the current working directory

Relative path:
- starts from some given working directory

Question 4

What does the built-in function “open” do in Python?

Answer 4

It opens a file and returns its filehandle.

Question 5

What functions of a filehandle have we learned about?

Answer 5

• filehandle.seek()
• filehandle.read()
• filehandle.write()

Question 6

What does the function filehandle.seek() do?

Answer 6

It sets the file’s cursor at a specified position in the current file.
It uses two arguments:
- offset: This is the number of positions of the cursor to move within the file.
- whence (optional): This asks from where the offset should start counting: The start of the file (0 (default)), current position of the cursor (1) or the end of the file (2).

Question 7

What does the function filehandle.read() do?

Answer 7

Read the content of the file.

Question 8

What does the function filehandle.write(“text”) do?

Answer 8

Writes “text” to the file.

Question 9

What arguments does the built-in function “open” have?

Answer 9

• filename (str)
• mode (str)

Question 10

What values are valid for the parameter “mode” for the built-in function “open”?

Answer 10

• “r”: Read-only (fails if the file doesn’t exist)
• “w”: Write (creates new file if it doesn’t exist, deletes original content if file already exists)
• “a”: Append (like “write” but doesn’t delete original content if file already exists, appends new content instead)
• “r+”: Read and Write
• “a+”: Read and Append

Question 11

What is the default cursor position for the following modes, when opening a file in Python using the built-in function “open”?

1. “r”
2. “w”
3. “a”
4. “r+”
5. “a+”

Answer 11

1. Beginning of file
2. Beginning of file
3. End of file
4. Beginning of file
5. End of file

Question 12

What does it mean if the letter “b” is added to the mode parameter of the build-in function “open” in Python?

Answer 12

It specifies that the file to open is not a text file and that it should be opened in binary mode. It won’t be interpreted that way and it only expects and returns bytes objects (instead of string objects).

Question 13

What can happen if you don’t close your file after opening it in Python?

Answer 13

• Not all the data is written to the file because it’s buffered and only closing the file flushes the last of the data to the file.
• The file may get corrupted if the program terminates before closing the file.

Question 14

How can a file be closed in Python?

Answer 14

with the build-in function “close”.

Question 15

How can we assure an opened file is closed, even if the program is terminated unexpectedly?

Answer 15

• put the close-function is the finally-part of a try-finally block
• use a with-statement that automatically closes the file for you

Question 16

What’s the syntax of a with-statement to read a file?

Answer 16

with open(“filepath.txt”) as fh:
content_as_string = fh.read()

Question 17

Which of the following statements is correct?

1. The with-statement automatically closes an open file.
2. The with-statement needs a try-finally block to ensure any open file gets closed.
3. The with-statement handles any exception that may occur when opening a file.
4. The with-statement needs to be put inside a try-except block to handle any exception that may occur when opening a file.

Answer 17

1 and 4 are correct

Question 18

Thinking of different OSs, what is the most flexible way to write a file path?

Answer 18

os.path.join(“folder”, “file.txt”)

This creates a compatible path according to the used OS.

Question 19

How can we store Python objects in files?

Answer 19

using the pickle or dill modules.

Question 20

What is the pickle module?

Answer 20

• allows to store and load Python objects in form of binary files
• doesn’t compress the files
• can handle many different Python objects

Question 21

What is the dill module?

Answer 21

• has the same interface as the pickle module
• extends the pickle module
• can handle more types of Python objects
• should be used instead of the pickle module
• often used like this: import dill as pickle