io.RawIOBase.read()

read(size=-1)

Read up to size bytes from the object and return them. As a convenience, if size is unspecified or -1, readall() is called. Otherwise, only one system call is ever made. Fewer than size bytes may be returned if the operating system call returns fewer than size bytes.

If 0 bytes are returned, and size was not 0, this indicates end of file. If the object is in non-blocking mode and no bytes are available, None is returned.