readuntil

Readuntil is a coroutine method of asyncio.StreamReader in Python’s standard library. It reads data from an asynchronous stream until the specified separator bytes are encountered, returning the accumulated data including the separator.

If the end of the stream is reached before the separator is found, readuntil raises asyncio.IncompleteReadError, which carries any partial data that was read. If the search exceeds the configured limit without locating the separator, it raises asyncio.LimitOverrunError. The method respects the StreamReader’s limit, which defaults to 65536 bytes but can be adjusted when creating the reader or by setting the limit parameter.

Usage typically appears in async protocols where messages are delimited by a known sequence, such as a newline or a custom delimiter. For example, data = await reader.readuntil(b'\n') reads a single line terminated by a newline. The returned bytes include the delimiter, and decoding to text is usually performed after retrieval, e.g., line = data.decode().

Relation to other methods: readuntil differs from readexactly in that it waits for a delimiter rather than a fixed byte count, and it differs from readline in that the delimiter can be any specified sequence rather than only a newline. Readuntil is part of the broader asyncio Streams API available in Python 3.5 and later, and is commonly used in network clients and servers that exchange delimited messages.

Limitations and considerations: care is needed to ensure the chosen delimiter reliably appears in the stream and to handle possible exceptions (IncompleteReadError, LimitOverrunError) in error handling paths.