Python-сообщество

пишу:

pi = 3.14
s = pi + 2
l = len(str(s))
print("pi =", pi, ",", "s =", s, l)

3.4.2 (v3.4.2:ab2c023a9432, Oct 6 2014, 22:16:31)
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pi = 3.14 , s = 5.140000000000001 17

https://docs.python.org/2/tutorial/floatingpoint.html#representation-error

уже отвечали тут http://python.su/forum/topic/26534/

It’s important to realize that this is, in a real sense, an illusion: the value in the machine is not exactly 1/10, you’re simply rounding the display of the true machine value. This fact becomes apparent as soon as you try to do arithmetic with these values

>>> 0.1 + 0.2
0.30000000000000004

ну и значение Pi можно брать из модуля math, math.pi