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[Python-Dev] Assignment expression and coding style: the while True case

MRAB python at mrabarnett.plus.com
Wed Jul 4 21:33:44 EDT 2018 
On 2018-07-05 01:11, Victor Stinner wrote:
> 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.
In this case, the second loop might be better left as-is because there 
are 2 conditions for leaving the loop. Stylistically, it might be 
starting to hurt readability with something like:

while avail < n or (chunk := self.raw.read(wanted)) not in empty_values:

[snip]
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