>>> from collections import defaultdict
>>> d = defaultdict(int)
>>> for x in [1, 2, 3, 1, 2, 3, 1]:
...     d[x] += 1
...
>>> dict(d)
{1: 3, 2: 2, 3: 2}
>>>
>>> c = defaultdict(int)
>>> for y in ['a', 'b', 'a', 'c', 'c']:
...     c[y] += 1
...
>>> dict(c)
{'a': 2, 'c ...

>>> def foo():
...     return
...     yield
...
>>> foo()
<generator object foo at 0x01C46968>
>>> list(foo())
[]
>>>

# 创建大小为 10M 的文件：abc.exe
dd if=/dev/zero of=abc.exe bs=1M count=10

# 只生成属性为 10M 大小的文件，但不占用实际硬盘空间
dd if=/dev/zero of=abc.exe bs=1M seek=10 count=0