如何将其转换为Python列表推导？

Question

I'd like to make this more efficient but I can't figure out how to turn this into a python list comprehension.

coupons = []
for source in sources:
    for coupon in source:
        if coupon.code_used not in coupons:
            coupons.append(coupon.code_used)

Answer 1

您无法访问当前创建的列表，但是如果顺序不重要，则可以使用set ：

coupons = set(coupon.code_used for source in sources for coupon in source)

Answer 2

used_codes = set(coupon.code_used for source in sources for coupon in source)

Answer 3

I'm going to assume that the order of the resulting list is unimportant, because then we can just use a set to eliminate duplicates.

coupons = list(set(coupon.code_used for source in sources for coupon in source))

This uses a generator expression, with the for clauses appearing in the same order as in the nested loop, to extract all the codes. The set keeps only unique codes, and list creates a list (arbitrarily ordered) from the set.

Answer 4

coupons = {x for x in (y.code_used for y in coupon for coupon in sources)}

您实际上正在寻找set