Numba, a just-in-time compiler for Python, transforms Python code into machine code for optimized execution. This process, however, poses a challenge for traditional code coverage tools, as they typically operate within the Python interpreter and thus miss the lines of code compiled by Numba. To address this issue, Numba opts for a compile-time notification to coverage tools, rather than during execution, to minimize performance penalties. This approach helps prevent significant coverage gaps in projects utilizing Numba, without incurring substantial performance costs.
No additional effort is required to generate compile-time coverage data. By
running a Numba application under the coverage tool
(e.g. coverage run ...), the compiler automatically
detects the active coverage session and emits data accordingly. This mechanism
ensures that coverage data is generated seamlessly, without the need for manual
intervention.
The coverage data is emitted during the lowering phase, which involves the generation of LLVM-IR. This phase inherently excludes lines of code that are statically identified as dead code, ensuring that the coverage data accurately reflects the executable code paths.