CWE-197: Control rounding when converting to less precise numbers

While defensive coding requires enforcing types, it is important to make conscious design decisions on how conversions are rounded.

The example01.py code demonstrates how int() behaves differently to round().

example01.py:

""" Code Example """

print(int(0.5))   # prints 0
print(int(1.5))   # prints 1
print(int(1.45))  # prints 1
print(int(1.51))  # prints 1
print(int(-1.5))  # prints -1

print(round(0.5))   # prints 0
print(round(1.5))   # prints 2
print(round(1.45))  # prints 1
print(round(1.51))  # prints 2
print(round(-1.5))  # prints -2

print(type(round(0.5)))  # prints <class 'int'>

The build in round() does not allow to specify the type of rounding in use [python round( ) 2024]. In Python 3 the round() function uses “bankers’ rounding” (rounds to the nearest even number in case of ties). This is different to Python 2 which always rounds away from zero. Rounding provided by the decimal module allows a choice between 8 rounding modes [python decimal 2024]. Rounding in mathematics and science is not discussed here as it requires a deeper knowledge of computer floating-point arithmetic’s.

Non-Compliant Code Example (float to int)

In noncompliant01.py there is no conscious choice of rounding mode.

noncompliant01.py:

""" Non-compliant Code Example """

print(int(0.5))    # prints 0
print(int(1.5))    # prints 1
print(round(0.5))  # prints 0
print(round(1.5))  # prints 2

Compliant Solution (float to int)

Using the Decimal class from the decimal module allows more control over rounding by choosing one of the 8 rounding modes [python decimal 2024].

compliant01.py:

""" Compliant Code Example """
from decimal import Decimal, ROUND_HALF_UP, ROUND_HALF_DOWN
 
print(Decimal("0.5").quantize(Decimal("1"), rounding=ROUND_HALF_UP))  # prints 1
print(Decimal("1.5").quantize(Decimal("1"), rounding=ROUND_HALF_UP))  # prints 2
print(Decimal("0.5").quantize(Decimal("1"), rounding=ROUND_HALF_DOWN))  # prints 0
print(Decimal("1.5").quantize(Decimal("1"), rounding=ROUND_HALF_DOWN))  # prints 1

The .quantize(Decimal("1"), determines the precision to be integer and rounding=ROUND_HALF_UP determines the type of rounding applied. Specifying numbers as strings avoids issues such as floating-point representations in binary.

That Decimal can have unexpected results when operated without Decimal.quantize() on floating point numbers is demonstrated in example02.py.

example02.py:

# SPDX-FileCopyrightText: OpenSSF project contributors
# SPDX-License-Identifier: MIT
""" Code Example """
from decimal import ROUND_HALF_UP, Decimal

print(Decimal("0.10")) # prints 0.10
print(Decimal(0.10))   # prints 0.1000000000000000055511151231257827021181583404541015625
print(Decimal("0.10").quantize(Decimal("0.10"), rounding=ROUND_HALF_UP)) # prints 0.10
print(Decimal(0.10).quantize(Decimal("0.10"), rounding=ROUND_HALF_UP)) # prints 0.10

Initializing Decimal with an actual float, such as 0.10, and without rounding creates an unprecise number 0.1000000000000000055511151231257827021181583404541015625 in Python 3.9.2.

Automated Detection

Tool Version Checker Description
Bandit 1.7.4 on Python 3.10.4 Not Available  
Flake8 8-4.0.1 on Python 3.10.4 Not Available  
   
MITRE CWE Pillar CWE-682, Incorrect Conversion between Numeric Types (mitre.org)
MITRE CWE Class CWE-197, Numeric Truncation Error
SEI CERT C Coding Standard INT31-C. Ensure that integer conversions do not result in lost or misinterpreted data
SEI CERT C Coding Standard FLP34-C. Ensure that floating-point conversions are within range of the new type
ISO/IEC TR 24772:2019 Programming languages — Guidance to avoiding vulnerabilities in programming languages, available from https://www.iso.org/standard/71091.html
SEI CERT Coding Standard for Java NUM12-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data

Biblography

   
python round( ) 2024 python round( ), available from: https://docs.python.org/3/library/functions.html#round, [Last accessed June 2024]
python decimal 2024 Python decimal module, available from: https://docs.python.org/3/library/decimal.html#rounding-modes
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