marius311 / Py2Call

Call Python 2 and Python 3 from the same Julia session

Py2Call.jl

Call both Python 2 and Python 3 from a single Julia session.

Install

julia> ENV["PYTHON"] = "/path/to/python"
julia> ENV["PYTHON2"] = "/path/to/python2"
pkg> add https://github.com/marius311/Py2Call.git

Usage

julia> using PyCall

julia> using Py2Call

julia> py"""
       import sys
       """

julia> py2"""
       import sys
       """

julia> py"sys.version"
"3.7.3 (default, Apr  4 2019, 23:33:31) \n[GCC 7.3.0]"

julia> py2"sys.version"
"2.7.16 (default, Apr  6 2019, 01:42:57) \n[GCC 8.3.0]"

You can always reset the Python versions used,

PYTHON=/path/to/python PYTHON2=/path/to/python2 julia -e 'using Pkg; Pkg.build("Py2Call")'

If you only use Python 2 from your script, you can do using Py2Call: @py_str and then use py"..." which is more succinct and may be syntax highlighted depending on editor, but will call Python 2.

Limitations

• PyCall creates a Python namespace for each Julia module, but Py2Call will always evaluate in the Main namespace regardless which Julia module initiates the call.

• The return value of py2"..." expressions must be picklable, or auto-convert to Julia types which are serializable.

How it works

Building this package installs an older version of PyCall (1.91.3) into it's own environment and builds it for Python 2, while the version of PyCall in your main environment stays built for Python 3 (note, this means you can't use 1.91.3 in your main environment).

The Py2Call package then spawns a worker process which is running in the other environment, and py2"..." simply forwards to py"..." to the worker. Note two consequences of this extra worker: 1) All using Foo statements after you load Py2Call will load Foo on the remote worker as well, and thus take longer; put using Py2Call at the end of your imports to remedy this 2) If you run parallel jobs using all workers, one of the workers will be the Python 2 worker. If this is undesired, exclude it; you can get its worker ID with Py2Call.id_py2worker.