use system python if condapkg backend is null

Author: Christopher Doris

src/cpython/context.jl

@@ -52,33 +52,41 @@ function init_context()
         # Find Python executable
         exe_path = get(ENV, "JULIA_PYTHONCALL_EXE", "")
         if exe_path == "" || exe_path == "@CondaPkg"
-            if Sys.islinux()
-                # Ensure libstdc++ in the Conda environment is compatible with the one
-                # linked in Julia. This is platform/version dependent, so needs to occur at
-                # runtime.
-                #
-                # To figure out cxx_version for a given Julia version, run
-                #   strings /path/to/julia/lib/julia/libstdc++.so.6 | grep GLIBCXX
-                # then look at
-                #   https://gcc.gnu.org/onlinedocs/gcc-12.1.0/libstdc++/manual/manual/abi.html
-                # for the highest GCC version compatible with the highest GLIBCXX version.
-                if Base.VERSION <= v"1.6.2"
-                    # GLIBCXX_3.4.26
-                    cxx_version = ">=3.4,<9.2"
-                else
-                    # GLIBCXX_3.4.29
-                    # checked up to v1.8.0
-                    cxx_version = ">=3.4,<11.4"
+            if CondaPkg.backend() == :Null
+                exe_path = Sys.which("python")
+                if exe_path === nothing
+                    error("CondaPkg is using the Null backend but Python is not installed")
+                end
+                exe_path::String
+            else
+                if Sys.islinux()
+                    # Ensure libstdc++ in the Conda environment is compatible with the one
+                    # linked in Julia. This is platform/version dependent, so needs to occur at
+                    # runtime.
+                    #
+                    # To figure out cxx_version for a given Julia version, run
+                    #   strings /path/to/julia/lib/julia/libstdc++.so.6 | grep GLIBCXX
+                    # then look at
+                    #   https://gcc.gnu.org/onlinedocs/gcc-12.1.0/libstdc++/manual/manual/abi.html
+                    # for the highest GCC version compatible with the highest GLIBCXX version.
+                    if Base.VERSION <= v"1.6.2"
+                        # GLIBCXX_3.4.26
+                        cxx_version = ">=3.4,<9.2"
+                    else
+                        # GLIBCXX_3.4.29
+                        # checked up to v1.8.0
+                        cxx_version = ">=3.4,<11.4"
+                    end
+                    CondaPkg.add("libstdcxx-ng", version=cxx_version, channel="conda-forge", temp=true, file=joinpath(@__DIR__, "..", "..", "CondaPkg.toml"), resolve=false)
                 end
-                CondaPkg.add("libstdcxx-ng", version=cxx_version, channel="conda-forge", temp=true, file=joinpath(@__DIR__, "..", "..", "CondaPkg.toml"), resolve=false)
+                # By default, we use Python installed by CondaPkg.
+                exe_path = Sys.iswindows() ? joinpath(CondaPkg.envdir(), "python.exe") : joinpath(CondaPkg.envdir(), "bin", "python")
+                # It's not sufficient to only activate the env while Python is initialising,
+                # it must also be active when loading extension modules (e.g. numpy). So we
+                # activate the environment globally.
+                # TODO: is this really necessary?
+                CondaPkg.activate!(ENV)
             end
-            # By default, we use Python installed by CondaPkg.
-            exe_path = Sys.iswindows() ? joinpath(CondaPkg.envdir(), "python.exe") : joinpath(CondaPkg.envdir(), "bin", "python")
-            # It's not sufficient to only activate the env while Python is initialising,
-            # it must also be active when loading extension modules (e.g. numpy). So we
-            # activate the environment globally.
-            # TODO: is this really necessary?
-            CondaPkg.activate!(ENV)
             CTX.which = :CondaPkg
         elseif exe_path == "@PyCall"
             # PyCall compatibility mode