问题描述:

My code:

`#!/usr/bin/python`

# -*- coding: utf-8 -*-

print (round(1.555,1)) #It seems normal

print (round(1.555,2)) #Why it is not output 1.56?

print (round(1.556,2)) #It seems normal

Output:

`[email protected]:~/code/python$ ./t2.py`

1.6

1.55

1.56

[email protected]:~/code/python$

`round(1.555,1)`

outputs `1.6`

.

Why doesn't `round(1.555,2)`

output `1.56`

?

Take a look at the documentation:

NoteThe behavior of`round()`

for floats can be surprising: for example,`round(2.675, 2)`

gives`2.67`

instead of the expected`2.68`

. This is not a bug: it’s a result of the fact that most decimal fractions can’t be represented exactly as a float. SeeFloating Point Arithmetic: Issues and Limitationsfor more information.

If you keep digging (i.e. click that link), you'll find an example similar to yours:

The documentation for the built-in

`round()`

function says that it rounds to the nearest value, rounding ties away from zero. Since the decimal fraction`2.675`

is exactly halfway between`2.67`

and`2.68`

, you might expect the result here to be (a binary approximation to)`2.68`

. It’s not, because when the decimal string`2.675`

is converted to a binary floating-point number, it’s again replaced with a binary approximation, whose exact value is`2.67499999999999982236431605997495353221893310546875`

String formatting won't fix your problem either. The floating point number just isn't stored the way you'd expect it to be:

```
>>> '{:0.2f}'.format(1.555)
'1.55'
```

This isn't really a "fix", but Python does have a `decimal`

module, which is designed for floating point arithmetic:

```
>>> from decimal import Decimal
>>> n = Decimal('1.555')
>>> round(n, 2)
Decimal('1.56')
```

Straight from the docs:

The behavior of round() for floats can be surprising: for example, round(2.675, 2) gives 2.67 instead of the expected 2.68. This is not a bug: it’s a result of the fact that most decimal fractions can’t be represented exactly as a float. See Floating Point Arithmetic: Issues and Limitations for more information.

From http://docs.python.org/2/library/functions.html#round:

Note

The behavior of round() for floats can be surprising: for example, round(2.675, 2) gives 2.67 instead of the expected 2.68. This is not a bug: it’s a result of the fact that most decimal fractions can’t be represented exactly as a float. See Floating Point Arithmetic: Issues and Limitations for more information.

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