为什么对于（看似）相同的代码我得到不同的输出？

Question

I'm trying to understand why these two examples are giving me different outputs.

Example1:

list1 = [1,2,3,4,5]
list2 = []
for l in list1:
    list2.append(l)
print list2

#[1, 2, 3, 4, 5]

Example2:

list1 = [1,2,3,4,5]
list2 = []
list2.append(l for l in list1)
print list2

#[<generator object <genexpr> at 0x10379ecd0>]

I've tried putting list() or tuple() after the append in the second example, but it's giving me one single element in the new list as opposed to 5 different ones.

list2.append(tuple(l for l in list1))

#[(1, 2, 3, 4, 5)]

Would there be a way for me to get same same output from Example1 using just one line for the for loop?

I'd really appreciate any help!

Answer 1

You need extend instead of append for the second example

list1 = [1,2,3,4,5]
list2 = []
list2.extend(l for l in list1)  # or just list2.extend(list1)
print list2

If all you want to do is copy list1 then

list2 = list1[:]

or

list2 = list(list1)

will do it

Answer 2

(l for l in list1) is a generator expression . It explicitly returns a generator, which is essentially a memory-saving iterator (think a list, but with each element returned on the fly, rather than stored all in memory). The key to realising it is a generator expression is in noting that the expression is surrrounded by ( and ) .

In list2.append(l for l in list1) , you have appended a single generator to the empty list. You can still get all the elements you want by doing for element in list2[0] , but it is not the same thing.

However, in doing

for l in list1:
   list2.append(l)

no such generator expressions are encountered. At every iteration - every run of the loop - l , an element, is appended to the end. Hence why this works the way you think.

---

If you want to get your list2 using something similar to the generator expression, you want to use a list comprehension instead:

list2 = [l for l in list1]

Note that the difference between the structure of a list comprehension and a generator expression is simply the presence of square brackets [ , ] rather than parentheses.

Answer 3

You don't want to use append in the second example. Instead you want

list2 = [l for l in list1]

There are easier ways to copy a list, but this is the one that most closely matches what you're going for in Example 2.

Answer 4

If you don't like the nested list then you may simply use :

list1 = [1,2,3,4,5]
list2 = []
[list2.append(l) for l in list1]