Rejection Notice

This PEP has been rejected. After sitting open for four years, it hasfailed to generate sufficient community interest.

Several ideas of this PEP were implemented for Python 2.6.float('inf')andrepr(float('inf')) are now guaranteed to work on every supportedplatform with IEEE 754 semantics. However theeval(repr(float('inf')))roundtrip is still not supported unless you define inf and nan yourself:

>>>inf=float('inf')>>>inf,1E400(inf, inf)>>>neginf=float('-inf')>>>neginf,-1E400(-inf, -inf)>>>nan=float('nan')>>>nan,inf*0.(nan, nan)

The math and the sys module also have gained additional features,sys.float_info,math.isinf,math.isnan,math.copysign.

Abstract

This PEP proposes an API and a provides a reference module thatgenerates and tests for IEEE 754 double-precision special values:positive infinity, negative infinity, and not-a-number (NaN).

Rationale

The IEEE 754 standard defines a set of binary representations andalgorithmic rules for floating point arithmetic. Included in thestandard is a set of constants for representing special values,including positive infinity, negative infinity, and indeterminate ornon-numeric results (NaN). Most modern CPUs implement theIEEE 754 standard, including the (Ultra)SPARC, PowerPC, and x86processor series.

Currently, the handling of IEEE 754 special values in Python dependson the underlying C library. Unfortunately, there is littleconsistency between C libraries in how or whether these values arehandled. For instance, on some systems “float(‘Inf’)” will properlyreturn the IEEE 754 constant for positive infinity. On many systems,however, this expression will instead generate an error message.

The output string representation for an IEEE 754 special value alsovaries by platform. For example, the expression “float(1e3000)”,which is large enough to generate an overflow, should return a stringrepresentation corresponding to IEEE 754 positive infinity. Python2.1.3 on x86 Debian Linux returns “inf”. On Sparc Solaris 8 withPython 2.2.1, this same expression returns “Infinity”, and onMS-Windows 2000 with Active Python 2.2.1, it returns “1.#INF”.

Adding to the confusion, some platforms generate one string onconversion from floating point and accept a different string forconversion to floating point. On these systems

float(str(x))

will generate an error when “x” is an IEEE special value.

In the past, some have recommended that programmers use expressionslike:

PosInf=1e300**2NaN=PosInf/PosInf

to obtain positive infinity and not-a-number constants. However, thefirst expression generates an error on current Python interpreters. Apossible alternative is to use:

PosInf=1e300000NaN=PosInf/PosInf

While this does not generate an error with current Pythoninterpreters, it is still an ugly and potentially non-portable hack.In addition, defining NaN in this way does solve the problem ofdetecting such values. First, the IEEE 754 standard provides for anentire set of constant values for Not-a-Number. Second, the standardrequires that

NaN!=X

for all possible values of X, including NaN. As a consequence

NaN==NaN

should always evaluate to false. However, this behavior also is notconsistently implemented. [e.g. Cygwin Python 2.2.2]

Due to the many platform and library inconsistencies in handling IEEEspecial values, it is impossible to consistently set or detect IEEE754 floating point values in normal Python code without resorting todirectly manipulating bit-patterns.

This PEP proposes a standard Python API and provides a referencemodule implementation which allows for consistent handling of IEEE 754special values on all supported platforms.

API Definition

>>>importfpconst>>>val=1e30000# should be cause overflow and result in "Inf">>>valInfinity>>>fpconst.isInf(val)1>>>fpconst.PosInfInfinity>>>nval=val/val# should result in NaN>>>nvalNaN>>>fpconst.isNaN(nval)1>>>fpconst.isNaN(val)0

References

Seehttp://babbage.cs.qc.edu/courses/cs341/IEEE-754references.html forreference material on the IEEE 754 floating point standard.

Copyright

This document has been placed in the public domain.