列表解析中的循环是从左到右读的。如果你的列表解析被写成一个普通的循环，它看起来会像这样：

>>> vec = [[1,2,3], [4,5,6], [7,8,9]]
>>> l = []
>>> for elem in vec:
...     for num in elem:
...         l.append(num)
...
>>> l

[1, 2, 3, 4, 5, 6, 7, 8, 9]

在Python 2中，列表解析中的变量共享外部作用域，所以num可以在后面使用：

>>> vec = [[1,2,3], [4,5,6], [7,8,9]]

>>> [num for elem in vec for num in elem]
[1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> num
9

注意，在Python 3中，行为是不同的：

>>> vec = [[1,2,3], [4,5,6], [7,8,9]]

>>> [num for elem in vec for num in elem]
[1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> num
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'num' is not defined

当执行列表解析时，num的值是9，所以下次迭代vec时，将得到9的列表。
看这个。

In [1]: vec = [[1,2,3], [4,5,6], [7,8,9]]

In [2]: [num for elem in vec for num in elem]
Out[2]: [1, 2, 3, 4, 5, 6, 7, 8, 9]

In [3]: num
Out[3]: 9

In [4]: [num for elem in vec]
Out[4]: [9, 9, 9]

让我试着把答案说得更清楚些，顺序显然是从左到右，最右边的值将存储在变量中，即 num 和 elem。
初始数据：

vec = [[1,2,3], [4,5,6], [7,8,9]]
num  # Undefined
elem # Undefined

第1步：执行[num for elem in vec for num in elem]行后

# Now the value of undefined variable will be
num = 9  # Will keep the last value of the loop as per Python 2.7
elem = [7, 8, 9]   # The same applies here (as the loop
                   # for 'elem' in 'vec' will get
                   # executed first, followed by
                   # for 'num' in 'elem')

第2步：执行[num for elem in vec]行后

# Output: [9, 9, 9]
# Since num value is 9 and its get repeated 3 times because of
# 'vec' has three elements (three list object in a list, so
# the 'for' loop will run three times)
# Now the variable value would be
num = 9 # No change
elem = [7, 8, 9] # The last tuple of variable vec

步骤3：执行[num for elem in (vec for num in elem)]之后
1.在第一个/右侧循环中，即（vec for num in elem）
这里的结果将是一个生成器，由于 elem 的长度为3，它将运行三次。
1.最后的 for 循环将在RESULT 1（长度为3的 for 循环#1的结果）上迭代，并且由于 num 值为9，因此结果将为[9，9，9] # 'num' value重复三次。

list1 = [num for elem in vec for num in elem]

相当于：

list1 = []
for elem in vec:
    for num in elem:
       list1.append(num)