On 9/20/2016 11:51 AM, Guido van Rossum wrote:
We seem to have fundamentally different ideas of what sort of code is
most readable. The alternative to partial is usually a lambda,
Partial and lambda are different in that partial captures variable
values immediately, whereas lambda does not, unless the default argument
trick, with its attendant disadvantages, is used. I consider this one
reason to chose one or the other. (The greater flexibility of lambda,
pointed out by David Mertz, is another.)
# Demonstration
from functools import partial
def f(a): print(a)
# lambda
fl = []
for i in range(3):
fl.append(lambda: f(i))
fl[0]()
# 2 - bug
flc = [lambda: f(i) for i in range(3)]
flc[0]()
# 2 - bug
flcd = [lambda i=i: f(i) for i in range(3)]
flcd[0]()
# 0 - correct
# partial
fp = []
for i in range(3):
fp.append(partial(f, i))
fp[0]()
# 0 - correct
fpc = [partial(f, i) for i in range(3)]
fpc[0]()
# 0 - correct
and the
signature of the lambda helps me understand how it is being called.
The 'i=i' signature is typically left out by beginners creating multiple
functions in a loop, leading to one of *the* most frequent of FAQs.
https://docs.python.org/3/faq/programming.html#why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result
When added 'i=i', better written '_i=i', is misleading, as '_i' is not
really a parameter and 'i' is not a replaceable default value, but is
supposed to be a fixed value, as with partial.
I believe in a previous thread about creating functions in a loop, it
was generally agreed that using partial is better. (This alternative
has not made it to the FAQ yet, but I thing it should.)
I otherwise generally agree with what you wrote.
--
Terry Jan Reedy
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