codon/docs/intro/differences.md

While Codon's syntax and semantics are nearly identical to Python's, there are some notable differences that are worth mentioning. Most of these design decisions were made with the trade-off between performance and Python compatibility in mind.

Data types

• Integers: Codon's int is a 64-bit signed integer, whereas Python's (after version 3) can be arbitrarily large. However Codon does support larger integers via Int[N] where N is the bit width.

• Strings: Codon currently uses ASCII strings unlike Python's unicode strings.

• Dictionaries: Codon's dictionary type does not preserve insertion order, unlike Python's as of 3.6.

• Tuples: Codon's tuples are by-value objects (i.e., they are not references) and their size must be known at compile-time.

Type checking

Since Codon performs static type checking ahead of time, a few of Python's dynamic features are disallowed. For example, monkey patching classes at runtime (although Codon supports a form of this at compile time) or adding objects of different types to a collection.

These few restrictions are ultimately what allow Codon to compile to native code without any runtime performance overhead. Future versions of Codon will lift some of these restrictions by the introduction of e.g. implicit union types.

Numerics

For performance reasons, some numeric operations use C semantics rather than Python semantics. This includes, for example, raising an exception when dividing by zero, or other checks done by math functions. Strict adherence to Python semantics can be achieved by using the -numerics=py flag of the Codon compiler. Note that this does not change ints from 64-bit.

Modules

While most of the commonly used builtin modules have Codon-native implementations, a few are not yet implemented. However these can still be used within Codon via from python import.

Many other missing features are also described in [Roadmap].