Reading and writing binary files

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Quick Questions

Shade in the A or B circle for each question.

1. Which line opens a file so read() returns raw bytes?

A open('data.bin', 'rb')

B open('data.bin', 'r')

2. Which mode do you use to write raw bytes to a file?

A 'wb'

B 'w'

3. What type does f.read() return when f was opened with 'rb'?

A bytes

B str

4. Which write call works on a file opened with 'wb'?

A f.write('Hi')

B f.write(b'Hi')

5. A file is opened with open('pic.png', 'rb'). What does f.read(4) return?

A The first 4 lines of the file

B The first 4 bytes of the file

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1f = open('data.bin', 'rb')		AA str is written to a file in 'wb' mode. Binary mode needs bytes, so use b'Hello' or 'Hello'.encode().
2f = open('copy.bin', 'wb')		Bf.write(b'AB')
3Reading exactly 4 bytes from a file opened in binary mode		COpens the file in binary read mode so reading gives raw bytes instead of text
4Binary data is type bytes, not str, so a b prefix is used for a bytes literal		DThe file is opened in text mode 'r', so read(4) gives 4 characters as str, not 4 raw bytes. Use 'rb'.
5f = open('data.bin', 'wb') f.write('Hello')		EOpens the file in binary write mode so you can write raw bytes to it
6f = open('data.bin', 'r') data = f.read(4)		Fchunk = f.read(4)

Definitions

open('data.bin', 'rb')

Fill in the Blanks

Complete each line of code by filling in the missing parts.

with open('data.bin', '__') as f: data = f.read() print(type(data))

with open('copy.bin', '__') as f: f.write(bytes([65, 66, 67]))

with open('data.bin', 'rb') as f: header = f.____(4) print(header)

with open('data.bin', 'rb') as f: data = f.read() print(isinstance(data, _____))

with ____('data.bin', 'rb') as f: chunk = f.read(8) print(len(chunk))

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A f = open('data.bin', 'rb')

B data = f.read[4]

C print(data)

D f.close()

Question 2

A f = open('out.bin', 'wb')

B data = bytes([72, 105])

C f.write(str(data))

D f.close()

Question 3

A with open('a.png', 'rb') as f:

B header = f.read(8)

C print(header + '!')

D print(type(header))

Question 4

A text = "Hello"

B data = text.decode()

C f = open('msg.bin', 'wb')

D f.write(data)

Question 5

A with open('d.bin', 'rb') as f:

B chunk = f.read(16)

C while chunk:

D print(chunk.length)

E chunk = f.read(16)

Answer Key

Reading and writing binary files

Section A: Quick Questions

A — open('data.bin', 'rb')

A — 'wb'

A — bytes

B — f.write(b'Hi')

B — The first 4 bytes of the file

Section B: Match the Pairs

f = open('data.bin', 'rb') → Opens the file in binary read mode so reading gives raw bytes instead of text

f = open('copy.bin', 'wb') → Opens the file in binary write mode so you can write raw bytes to it

Reading exactly 4 bytes from a file opened in binary mode → chunk = f.read(4)

Binary data is type bytes, not str, so a b prefix is used for a bytes literal → f.write(b'AB')

f = open('data.bin', 'wb') f.write('Hello') → A str is written to a file in 'wb' mode. Binary mode needs bytes, so use b'Hello' or 'Hello'.encode().

f = open('data.bin', 'r') data = f.read(4) → The file is opened in text mode 'r', so read(4) gives 4 characters as str, not 4 raw bytes. Use 'rb'.

Section C: Definitions

open('data.bin', 'rb') — Opens a file in binary mode so read() returns raw bytes, not text. Use 'wb' to write bytes. Binary data is type bytes, not str, and read(n) reads n bytes.

Section D: Fill in the Blanks

with open('data.bin', 'rb') as f:
    data = f.read()
    print(type(data))

with open('copy.bin', 'wb') as f:
    f.write(bytes([65, 66, 67]))

with open('data.bin', 'rb') as f:
    header = f.read(4)
    print(header)

with open('data.bin', 'rb') as f:
    data = f.read()
print(isinstance(data, bytes))

with open('data.bin', 'rb') as f:
    chunk = f.read(8)
    print(len(chunk))

Section E: Spot the Error

B — read is a method, so the number of bytes goes in round brackets: f.read(4). Square brackets are for indexing, not for calling a method.

C — A file opened with 'wb' only accepts bytes. str(data) turns it into text, which raises a TypeError. It should be f.write(data).

C — header is bytes, and bytes cannot be joined to a str. Use a bytes literal instead: header + b'!'.

B — A str is turned into bytes with encode(). decode() goes the other way, from bytes to str, and str has no decode method. It should be text.encode().

D — bytes objects have no .length attribute. Use the len() function to count the bytes: print(len(chunk)).