Skip to content

Latest commit

 

History

History
37 lines (25 loc) · 2.31 KB

python.md

File metadata and controls

37 lines (25 loc) · 2.31 KB
Raw

Imports

Python supports "relative" and "absolute" imports, but there's a catch: "absolute imports" may actually be local files. I.e. they don't have to be system imports; and as a matter of fact, you should not use "relative" ones at all (more below)

  • absolute imports:
    • import foo
    • from foo import bar
  • relative imports
    • from . foo import bar
    • from . foo import foo
    • WILL NOT WORK: import . foo.

There are 2 problems with relative imports that I know of:

  • they badly designed: for some reason, if you call main script file from a different directory, the local imports will no longer be found (even though they supposed to be relative to the importer's dir, i.e. disregarding where main file is being located).
  • mypy is gonna complain that the files hasn't opted-in to typing-system. Although that one is clearly a mypy problem, and there are workarounds, but together with the prev. point that increases questionability of the feature.

Instead you should import local files with absolute path. So, given this tree:

main.py
dir/myimport.py
dir/myfile.py

The myfile.py should import the myimport as import dir.myimport. That utilizes PEP420, and should work in modern python3. This absolute import is truly relative (to main.py), and you may call it as ./main.py, /tmp/main.py, etc, the import always just works.

Mypy has by default PEP420 disabled, you need to enable it with namespace_packages = True and explicit_package_bases = True options.

Trivia

  • paths to evaluate "absolute" imports against are stored in sys.path (import sys), which can be modified at runtime. It's a list that has one special element '', an empty string, which means "use the location of the script as one of directories for absolute imports".

Misc

  • pytest is better be called explicitly in the file that invokes test, rather than by a user on command line. The reason is that pytest binary name varies on different systems, it may be pytest3 or something else… It's better to just call pytest.main() from the testing file explicitly.
    • example of calling it on the current file: exit(pytest.main(argv)). This way argv[0] gets passed as the script name, so pytest would run tests defined in the same file. Plus any additional args such as -vv, etc, will be passed to pytest as well.