Compare decimal and float

Classification

• Affected Components : builtin compare, decimal

• Operating System : Linux

• Python Versions : 2.6.x, 2.7.x, 3.1.x, 3.2.x

• Reproducible : Yes

Source code

from decimal import Decimal

# Should report error as we are comparing FLOAT to DECIMAL that are DIFFERENT TYPES
print("Content of Object1 is FLOAT '1172837167.27'")
print("Content of Object2 is DECIMAL '1172837167.27'")
try:
    x = Decimal("1172837136.0800")
    y = 1172837136.0800
    if y > x:
        print("ERROR: FLOAT is detected as same as DECIMAL")
    else:
        print("ERROR: FLOAT seems comparable with DECIMAL")
except Exception as e:
    print("OK: FLOAT is NOT comparable with DECIMAL")

# Output:
# Python 2.6.5 32bit -- ERROR: FLOAT seems comparable with DECIMAL   (WRONG)
# Python 2.7.4 32bit -- ERROR: FLOAT seems comparable with DECIMAL   (WRONG)
# Python 3.1.2 32bit -- OK: FLOAT is NOT comparable with DECIMAL   (CORRECT)

Steps to Produce/Reproduce

To reproduce the problem copy the source code in a file and execute the script using the following command syntax:

$ python -OOBRtt test.py

Alternatively you can open python in interactive mode:

$ python -OOBRtt <press enter>

Then copy the lines of code into the interpreter.

Description

Python internal compare function does not verify if a comparison of two objects is done by using object of the same type.

In this case python does not know how to compare DECIMAL and FLOATS and returns a FALSE instead of returning an Error.

In this case we have asked python to compare a an object of type DECIMAL:

    x = Decimal("1172837136.0800")

with an object of type FLOAT:

    y = 1172837136.0800

and to make the comparison we introduced an if condition:

    if y > x:
        print("ERROR: FLOAT is detected as same as DECIMAL")
    else:
        print("ERROR: FLOAT seems comparable with DECIMAL")

Python should have strict rules to only allow comparison between objects is aware of but depending on the version of Python we have different behaviors:

• Python 2.6.5 -- ERROR: FLOAT seems comparable with DECIMAL (WRONG)
• Python 2.7.4 -- ERROR: FLOAT seems comparable with DECIMAL (WRONG)

Both python 2.6 and python 2.7 are not able to make the comparison and the 'fail open' logic returns a FALSE that produce the odd situation in which seems that DECIMAL and FLOAT are comparable.

• Python 3.1.2 -- OK: FLOAT is NOT comparable with DECIMAL (CORRECT)

Python 3.1 and newer is able to discriminate between object types and return an error that is intercepted by the exception:

except Exception as e:
    print("OK: FLOAT is NOT comparable with DECIMAL")

And therefore a message is printed to state that the comparison failed as expected with an error.

Workaround

Internal function able to properly discriminate between object types are available in Python 3.1 and later but not in any previous version.

Secure Implementation

WORK IN PROGRESS

References

[Python Built-in types][01] [01]:https://docs.python.org/2/library/stdtypes.html

[Python Introduction][02] [02]:https://docs.python.org/2/tutorial/introduction.html

[Python Data Model][03] [03]:https://docs.python.org/2/reference/datamodel.html

[Python 'decimal' module][04] [04]:https://docs.python.org/2/library/decimal.html