774 v2 (#779)

* 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.

* change to just one new option (lib) with sensible discovery/defaults of libpath, exepath and bindir depending on if others are set

* remove 'if' that is always true

---------

Co-authored-by: Nicolae Cudlenco <146981376+ncudlenco@users.noreply.github.com>
Co-authored-by: Christopher Rowley <github.com/cjdoris>

Author: cjdoris

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Changelog
 
 ## Unreleased
+* Added option `lib` to JuliaCall. Setting this will skip the discovery subprocess.
 * Bug fixes.
 
 ## 0.9.34 (2026-05-18)

docs/src/juliacall.md

@@ -142,6 +142,7 @@ 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-lib=<file>` | `PYTHON_JULIACALL_LIB=<file>` | Path to libjulia. Set to skip the relatively slow discovery process. |
 | `-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

@@ -179,39 +179,45 @@ def args_from_config(config):
     CONFIG['opt_handle_signals'] = choice('handle_signals', ['yes', 'no'])[0]
     CONFIG['opt_startup_file'] = choice('startup_file', ['yes', 'no'])[0]
     CONFIG['opt_heap_size_hint'] = option('heap_size_hint')[0]
-    CONFIG['project'] = path_option('project', check_exists=True)[0]
-    CONFIG['exepath'] = executable_option('exe')[0]
+    CONFIG['project'] = project = path_option('project', check_exists=True)[0]
+    CONFIG['libpath'] = libpath = path_option('lib', check_exists=True)[0]
+    CONFIG['exepath'] = exepath = executable_option('exe')[0]
 
     # Stop if we already initialised
     if CONFIG['inited']:
         return
 
-    have_exepath = CONFIG['exepath'] is not None
-    have_project = CONFIG['project'] is not None
-    if have_exepath and have_project:
+    if (exepath is None) and (bindir is not None):
+        # if bindir is set then set exepath={bindir}/julia
+        CONFIG['exepath'] = exepath = os.path.join(bindir, 'julia.exe' if os.name == 'nt' else 'julia')
+    if (exepath is not None) and (project is not None):
         pass
-    elif (not have_exepath) and (not have_project):
+    elif (exepath is None) and (project is None):
         # we don't import this at the top level because it is not required when
         # juliacall is loaded by PythonCall and won't be available, or if both
         # `exepath` and `project` are set by the user.
         import juliapkg
 
         # Find the Julia executable and project
-        CONFIG['exepath'] = juliapkg.executable()
-        CONFIG['project'] = juliapkg.project()
+        CONFIG['exepath'] = exepath = juliapkg.executable()
+        CONFIG['project'] = project = juliapkg.project()
     else:
         raise Exception("Both PYTHON_JULIACALL_PROJECT and PYTHON_JULIACALL_EXE must be set together, not only one of them.")
-
-    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')
+    if (libpath is not None) and (exepath is None):
+        raise Exception("PYTHON_JULIACALL_EXE is required if PYTHON_JULIACALL_LIB is set.")
+
+    # Find the Julia library, if not specified.
+    if libpath is None:
+        cmd = [exepath, '--project='+project, '--startup-file=no', '-O0', '--compile=min',
+               '-e', 'import Libdl; print(abspath(Libdl.dlpath("libjulia")), "\\0", Sys.BINDIR)']
+        libpath, found_bindir = subprocess.run(cmd, check=True, capture_output=True, encoding='utf8').stdout.split('\0')
+        CONFIG['libpath'] = libpath
+        if bindir is None:
+            CONFIG['bindir'] = bindir = found_bindir
+    if bindir is None:
+        bindir = os.path.dirname(exepath)
     assert os.path.exists(libpath)
-    assert os.path.exists(default_bindir)
-    CONFIG['libpath'] = libpath
+    assert os.path.exists(bindir)
 
     # Add the Julia library directory to the PATH on Windows so Julia's system libraries can
     # be found. They are normally found because they are in the same directory as julia.exe,
@@ -242,7 +248,7 @@ def args_from_config(config):
     jl_init.argtypes = [c.c_char_p, c.c_char_p]
     jl_init.restype = None
     jl_init(
-        (default_bindir if bindir is None else bindir).encode('utf8'),
+        None if bindir is None else bindir.encode('utf8'),
         None if sysimg is None else sysimg.encode('utf8'),
     )