[Python-Dev] Assignment expression and coding style: the while True case (original) (raw)

Victor Stinner vstinner at redhat.com
Wed Jul 4 20:11:43 EDT 2018

The code comes from Lib/_pyio.py. Simplified code:

nodata_val = b"" ... if n is None or n == -1: ... current_size = 0 while True: chunk = self.raw.read() if chunk in empty_values: nodata_val = chunk break current_size += len(chunk) chunks.append(chunk) return b"".join(chunks) or nodata_val

... while avail < n: chunk = self.raw.read(wanted) if chunk in empty_values: nodata_val = chunk break avail += len(chunk) chunks.append(chunk)

... return out[:n] if out else nodata_val

It seems like "nodata_val = " assignment can be moved out of the first loop, but cannot be moved for the second loop (since the second loop has no iteration if "avail >= n").

Yeah, maybe for this specific file, assignment expressions could be used for the (C) case and would be worth it.

Victor

2018-07-05 1:49 GMT+02:00 MRAB <python at mrabarnett.plus.com>:

On 2018-07-04 23:51, Victor Stinner wrote: [snip]

(C) while True: chunk = self.raw.read() if chunk in emptyvalues: nodataval = chunk break ... "nodataval = chunk" cannot be put into the "chunk := self.raw.read()" assignment expression combined with a test. At least, I don't see how. If that's the only 'break' in the loop, then you know that 'chunk' will have an 'empty' value after the loop, so you can change it to: while True: chunk = self.raw.read() if chunk in emptyvalues: break ... nodataval = chunk which then leads to: while (chunk := self.raw.read()) not in emptyvalues: ... nodataval = chunk [snip]

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