feat(juliacall): allow supplying libjulia path / bindir to skip discovery subprocess

init() starts a short-lived Julia process solely to print libjulia's
path and Sys.BINDIR. In pre-built containers / system images these are
static and known ahead of time; allow supplying them via the libpath /
default_bindir options (PYTHON_JULIACALL_LIBPATH /
PYTHON_JULIACALL_DEFAULT_BINDIR) to skip the extra process. Behaviour is
unchanged unless both are set. Docs and CHANGELOG updated.

Author: ncudlenco

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+* JuliaCall: added `libpath` / `default_bindir` options
+  (`PYTHON_JULIACALL_LIBPATH` / `PYTHON_JULIACALL_DEFAULT_BINDIR`) to supply
+  libjulia's path and `Sys.BINDIR` directly, skipping the discovery subprocess
+  at startup. Behaviour is unchanged unless both are set.
+
 ## 0.9.34 (2026-05-18)
 * Bug fixes.
 

docs/src/juliacall.md

@@ -142,6 +142,8 @@ be configured in two ways:
 | `-X juliacall-heap-size-hint=<N>` | `PYTHON_JULIACALL_HEAP_SIZE_HINT=<N>` | Hint for initial heap size in bytes. |
 | `-X juliacall-exe=<file>` | `PYTHON_JULIACALL_EXE=<file>` | Path to Julia binary to use (overrides JuliaPkg). |
 | `-X juliacall-project=<dir>` | `PYTHON_JULIACALL_PROJECT=<dir>` | Path to the Julia project to use (overrides JuliaPkg). |
+| `-X juliacall-libpath=<file>` | `PYTHON_JULIACALL_LIBPATH=<file>` | Path to libjulia. If set together with `default-bindir`, skips the subprocess that discovers it. |
+| `-X juliacall-default-bindir=<dir>` | `PYTHON_JULIACALL_DEFAULT_BINDIR=<dir>` | Julia's `Sys.BINDIR`. If set together with `libpath`, skips the subprocess that discovers it. |
 | `-X juliacall-trace-compile=<stderr\|name>` | `PYTHON_JULIACALL_TRACE_COMPILE=<stderr\|name>` | Print precompile statements. |
 | `-X juliacall-trace-compile-timing` | `PYTHON_JULIACALL_TRACE_COMPILE_TIMING=<yes\|no>` | Include timings with precompile statements. |
 

pysrc/juliacall/__init__.py

@@ -205,10 +205,19 @@ def args_from_config(config):
     exepath = CONFIG['exepath']
     project = CONFIG['project']
 
-    # Find the Julia library
-    cmd = [exepath, '--project='+project, '--startup-file=no', '-O0', '--compile=min',
-           '-e', 'import Libdl; print(abspath(Libdl.dlpath("libjulia")), "\\0", Sys.BINDIR)']
-    libpath, default_bindir = subprocess.run(cmd, check=True, capture_output=True, encoding='utf8').stdout.split('\0')
+    # Find the Julia library.
+    #
+    # This normally starts a short-lived Julia process just to print libjulia's
+    # path and Sys.BINDIR. In deployment scenarios (e.g. a pre-built container
+    # or system image) these are static and known ahead of time, so they may be
+    # supplied directly via the `libpath` / `default_bindir` options to skip the
+    # extra process. Behaviour is unchanged unless both are set.
+    libpath = path_option('libpath', check_exists=True)[0]
+    default_bindir = path_option('default_bindir', check_exists=True)[0]
+    if libpath is None or default_bindir is None:
+        cmd = [exepath, '--project='+project, '--startup-file=no', '-O0', '--compile=min',
+               '-e', 'import Libdl; print(abspath(Libdl.dlpath("libjulia")), "\\0", Sys.BINDIR)']
+        libpath, default_bindir = subprocess.run(cmd, check=True, capture_output=True, encoding='utf8').stdout.split('\0')
     assert os.path.exists(libpath)
     assert os.path.exists(default_bindir)
     CONFIG['libpath'] = libpath