Text mode changes bytes on its way in and out. Binary mode changes nothing, which is the difference when the file is not text.
About 2 min of reading
Before this new lesson, one small question about the lesson you just finished. Recall it, do not look it up.
Check byte behaviour.
Bytes and first value.
Before you run it, say out loud what it will print. Then open the answer.
Length kept.
Before you run it, say out loud what it will print. Then open the answer.
Header bytes.
Before you run it, say out loud what it will print. Then open the answer.
Total size.
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.
Text mode is a translation layer. When the file is not text, that layer is a bug, so open binary and decode where you need words.
Anything that reports a size counts bytes. A text length can be smaller, which is where the two numbers disagree.
data = b"ab"
print(len(data), data[0])Binary gives bytes, not a string. Indexing gives a number, not a one letter string.
out = "a\r\nb"
print(len(out))Text mode can rewrite endings on write. Binary keeps exactly the bytes you gave.
data = b"\x89PNG"
print(data[:4])There is no encoding for an image. Opening it as text raises on the first bad byte.
data = b"\x00\x01\x02"
print(len(data))Byte length is the file size. A text length counts symbols, which can differ.
head = b"# note"
body = b"\x00\x01"
print(len(head) + len(body))A header of text plus a body of bytes needs binary mode and a decode for the header.
data = "hi".encode("utf-8")
print(data.decode("utf-8"))Decode once where text is needed. Keep bytes on the way through.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.