It Will Never Work in Theory

Polymorphism in Python

Posted Jun 13, 2016 by Greg Wilson

| Programming Languages |

Beatrice Åkerblom and Tobias Wrigstad: "Measuring Polymorphism in Python Programs". SPLASH'15, October 2015, https://people.dsv.su.se/~beatrice/python/dls15_large_images.pdf.

In a break from our usual practice of quoting abstracts, here are this paper's conclusions:

Our results show that while Python’s dynamic typing allows unbounded polymorphism, Python programs are predominantly monomorphic, that is, variables only hold values of a single type. This is true for program start-up and normal runtime, in library code and in program-specific code.

Nevertheless, most programs have a few places which are megamorphic, meaning that variables in those places contain values of many different types at different times or in different contexts. Smaller programs do not generally differ from larger programs in this.

Because of the high degree of monomorphism, most programs can be typed to a large extent using a very simple type systems. Our findings show that the receiver in 97.4% of all call-sites in the average program can be described by a single static type using a conservative nominal type system using single inheritance. If we add parametric polymorphism to the type system, we increase the typeability to 97.9% of all call-sites for the average program.

For clusters, the receiver objects are typeable using a conservative nominal type system using single inheritance to 95.6% (on average). If we instead use a structural type, the typeability increases somewhat to 96.7% (on average).

Most polymorphic and megamorphic parts of programs are not typeable by nominal or structural systems, for example due to use of value-based overloading. Structural typing is only slightly better than nominal typing at handling non-monomorphic program parts. This suggests that nominal and structural typing is not a deciding factor in type system design if typing polymorphic code is desirable. More powerful constructs are needed in these cases, such as refinement types.

In simple terms, only 2.5% of call sites in most programs can't be handled by name-based typing with single inheritance. Adding parametric polymorphism (i.e., generics) only makes half a percent of difference, and most of the remaining cases can't be handled by widely-used mechanisms. This doesn't mean that languages shouldn't include more complex type systems, but it does (or should) mean that the onus is on their designers to show that the complexity is worthwhile.

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