A file is bytes, and a program reads it as bytes when the text view is wrong. Reading bytes is how you handle any file whose format you know.
About 2 min of reading
Read a file as bytes.
Write three bytes and read them back.
Before you run it, say out loud what it will print. Then open the answer.
Count the bytes in the file.
Before you run it, say out loud what it will print. Then open the answer.
Read a two-byte header as hex.
Before you run it, say out loud what it will print. Then open the answer.
Read one byte and print its number.
Before you run it, say out loud what it will print. Then open the answer.
Before you write anything, put these lines back in the right order. Click a line to move it up one step. This support step is easier than writing code from scratch, and it is here to make the last example stick.
Read the first example one line at a time. A line that prints shows what appears on screen; the rest set things up quietly.
Opening a file in binary mode hands over the exact bytes. Text mode adds an encoding and line-ending rules, which is only right for text files.
A file whose format you know is read field by field: a header, then sized fields. That is how images, archives, and data files are parsed.
with open("/tmp/probe.bin", "wb") as f:
f.write(b"\x00\x01\x02")
with open("/tmp/probe.bin", "rb") as f:
print(f.read())Writing and reading in binary mode keeps the exact bytes. Text mode would change them.
with open("/tmp/probe.bin", "rb") as f:
print(len(f.read()))The size of a file is the count of its bytes. Reading in binary gives that count exactly.
with open("/tmp/probe.bin", "rb") as f:
head = f.read(2)
print(head.hex())Reading a fixed number of bytes first is how a format header is read. The rest follows from it.
with open("/tmp/probe.bin", "rb") as f:
print(f.read(1)[0])Reading one byte gives its number. Treating a file as text when it is binary is the mistake this avoids.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.