啊，Python 喜欢先试，后果另说
你大可以
try:
....n = float(input_str) # e.g. input_str = "123"
except ValueError:
....n = float("nan") # or any other default value you want

对了，作为豆知识
>>> float("inf")
inf
>>> float("-inf")
-inf

对了，突然想起来，也许这个才是 LZ 最想要的：
>>> from decimal import *
>>> setcontext(ExtendedContext)
>>> nan = Decimal("I'm not a number!")
>>> nan
Decimal('NaN')
>>> print nan
NaN

它还能做到更多：
>>> inf = Decimal(1) / Decimal(0)
>>> print inf
Infinity
>>> neginf = Decimal(-1) / Decimal(0)
>>> print neginf
-Infinity
>>> print inf + neginf
NaN
>>> print inf * neginf
-Infinity
>>> print 2 * inf
Infinity
>>> print 1 + nan
NaN