James February 2016

python reference about floating point number

Before question, Here are sample code. Take a look at those first,please.

>>> id(1)
1636939440
>>> a = 1
>>> b = 1
>>> c = 1
>>> id(a)
1636939440
>>> id(b)
1636939440
>>> id(c)
1636939440


>>> id("hello")
43566560
>>> a = "hello"
>>> b = "hello"
>>> c = "hello"
>>> id(a)
43566560
>>> id(b)
43566560
>>> id(c)
43566560



>>> id(3.14)
34312864
>>> a = 3.14
>>> b = 3.14
>>> c = 3.14
>>> id(a)
34312864
>>> id(b)
34312600
>>> id(c)
34312432

As you see above, in terms of Integer and String, Python variable references the object the same way. But floating point number works in different way. Why is that? Is there any special reason for that?

Answers


Nikita February 2016

For small integers and strings Python uses internal memory optimization. Since any variable in Python is a reference to memory object, Python puts such small values into the memory only once. Then, whenever the same value is assigned to any other variable, it makes that variable point to the object already kept in memory. This works for strings and integers as they are immutable and if the variable value changes, effectively it's the reference used by this variable that is changed, the object in memory with original value is not itself affected.

First of all, floating point numbers are not 'small', and, second, the same 3.14 in memory depending on calculations might be kept as 3.14123123456789 and 3.14123987654321 (just example numbers to explain). So these two values are two different objects, but during calculations and displaying the meaningful part looks the same, i.e. 3.14 (in fact there's obviously many more possible values in memory for the same floating point number). That's why reusing the same floating point number object in memory is problematic and doesn't worth it after all.

See more on how floating point numbers are kept in memory here:

http://floating-point-gui.de/

http://docs.python.org/2/tutorial/floatingpoint.html

Also, there's a big article on floating point numbers at Oracle docs.


Pouria Hadjibagheri February 2016

Mutable and immutable.

Strings, tuples and bytes are immutable, whilst lists and byte arrays are mutable. Read more about the concept here: Data models in Python.

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