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Fri Feb 23 23:41:28 EST 2018 · https://mail.python.org/pipermail/python-dev/2018-February/152276.html

Chris Barker writes:

 > But I still think the original:
 > 
 > [g(y) for x in range(5) for y in [f(x)]]
 > 
 > Is always going to be confusing to read.

But the point I was making with "def f(x=[0]):" was this: you have a
situation where your desired semantics is "value of some type"[1], but
the language's syntax doesn't permit a value of that type there, while
"singleton sequence of that type" works fine.

In fact, "singleton as value" is baked into Python in the form of
str.__getitem__ and bytes.__getitem__.  So we now have four use cases
for singleton as value: two stringish actual types, and the two idioms
"mutable default argument" and "local variable in comprehension".  The
horse is long since out of the barn.

Steve

Footnotes: 
[1]  Both "value" and "type" are used rather loosely here.

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[Python-Dev] The `for y in [x]` idiom in comprehensions