InsightSoftwareConsortium · hjmjohnson · Jun 23, 2026 · Jun 23, 2026 · Jun 30, 2026 · Jun 30, 2026
diff --git a/.github/workflows/arm.yml b/.github/workflows/arm.yml
@@ -80,6 +80,7 @@ jobs:
               BUILD_SHARED_LIBS:BOOL=OFF
               BUILD_EXAMPLES:BOOL=OFF
               ITK_WRAP_PYTHON:BOOL=ON
+              ITK_WRAP_CASTXML_CACHE:BOOL=ON
               ITK_USE_CLANG_FORMAT:BOOL=OFF
               ITK_COMPUTER_MEMORY_SIZE:STRING=11
             ctest-options: "-E itkPyBufferMemoryLeakTest"
@@ -102,6 +103,7 @@ jobs:
           echo "CCACHE_SLOPPINESS=pch_defines,time_macros" >> "$GITHUB_ENV"
           echo "CCACHE_DIR=${{ runner.temp }}/ccache" >> "$GITHUB_ENV"
           echo "CCACHE_MAXSIZE=5G" >> "$GITHUB_ENV"
+          echo "ITK_WRAP_CACHE=${{ runner.temp }}/itk-castxml-cache" >> "$GITHUB_ENV"
           if [ "$RUNNER_OS" == "Linux" ]; then
             sudo apt-get update -qq && sudo apt-get install -y ccache locales
             sudo locale-gen de_DE.UTF-8
@@ -118,6 +120,16 @@ jobs:
           restore-keys: |
             ccache-v4-${{ runner.os }}-${{ matrix.name }}-
 
+      - name: Restore CastXML cache
+        if: matrix.python-version != ''
+        id: restore-castxml-cache
+        uses: actions/cache/restore@v5
+        with:
+          path: ${{ runner.temp }}/itk-castxml-cache
+          key: castxml-v1-${{ runner.os }}-arm-python-${{ github.sha }}
+          restore-keys: |
+            castxml-v1-${{ runner.os }}-arm-python-
+
       - name: Restore ExternalData object store
         id: restore-externaldata
         uses: actions/cache/restore@v5
@@ -202,6 +214,13 @@ jobs:
           path: ${{ runner.temp }}/ccache
           key: ccache-v4-${{ runner.os }}-${{ matrix.name }}-${{ github.sha }}
 
+      - name: Save CastXML cache
+        if: ${{ !cancelled() && matrix.python-version != '' }}
+        uses: actions/cache/save@v5
+        with:
+          path: ${{ runner.temp }}/itk-castxml-cache
+          key: castxml-v1-${{ runner.os }}-arm-python-${{ github.sha }}
+
       # ExternalData object store is populated by
       # .github/workflows/populate-externaldata-cache.yml — a dedicated
       # workflow whose only job is to prefetch every CID and write the

diff --git a/CMake/itkWrapCastXMLCacheSupport.cmake b/CMake/itkWrapCastXMLCacheSupport.cmake
@@ -0,0 +1,58 @@
+set(
+  _ITK_WRAP_CASTXML_CACHE_SCRIPT_DEFAULT
+  "${ITK_SOURCE_DIR}/Wrapping/Generators/CastXML/itk-castxml-cache.py"
+)
+
+option(
+  ITK_WRAP_CASTXML_CACHE
+  "Use a content-addressed two-level cache for CastXML wrapping steps."
+  ON
+)
+mark_as_advanced(ITK_WRAP_CASTXML_CACHE)
+
+if(ITK_WRAP_CASTXML_CACHE)
+  set(
+    ITK_WRAP_CASTXML_CACHE_SCRIPT
+    "${_ITK_WRAP_CASTXML_CACHE_SCRIPT_DEFAULT}"
+    CACHE FILEPATH
+    "Path to the CastXML content-addressed cache wrapper script"
+  )
+  mark_as_advanced(ITK_WRAP_CASTXML_CACHE_SCRIPT)
+
+  if(NOT EXISTS "${ITK_WRAP_CASTXML_CACHE_SCRIPT}")
+    message(
+      FATAL_ERROR
+      "ITK_WRAP_CASTXML_CACHE is ON but the wrapper script was not found:\n"
+      "  ${ITK_WRAP_CASTXML_CACHE_SCRIPT}\n"
+      "Set ITK_WRAP_CASTXML_CACHE_SCRIPT to the correct path or turn off ITK_WRAP_CASTXML_CACHE."
+    )
+  endif()
+
+  if(NOT Python3_EXECUTABLE)
+    message(
+      FATAL_ERROR
+      "ITK_WRAP_CASTXML_CACHE requires Python3_EXECUTABLE to be set."
+    )
+  endif()
+
+  set(
+    ITK_WRAP_CASTXML_CACHE_MAX_DAYS
+    "13.9"
+    CACHE STRING
+    "Days before a CastXML cache entry is evicted by --evict after each build (default 13.9)"
+  )
+  mark_as_advanced(ITK_WRAP_CASTXML_CACHE_MAX_DAYS)
+
+  message(STATUS "CastXML content-addressed cache enabled")
+  message(STATUS "  Script: ${ITK_WRAP_CASTXML_CACHE_SCRIPT}")
+  message(
+    STATUS
+    "  Cache root: set ITK_WRAP_CACHE env var at build time (default: ~/.cache/itk-wrap)"
+  )
+  message(
+    STATUS
+    "  Eviction (after each build): entries older than ${ITK_WRAP_CASTXML_CACHE_MAX_DAYS} days, then oldest trimmed to ITK_WRAP_CACHE_MAX_SIZE GB (default 2.0)"
+  )
+endif()
+
+unset(_ITK_WRAP_CASTXML_CACHE_SCRIPT_DEFAULT)
diff --git a/Documentation/docs/contributing/index.md b/Documentation/docs/contributing/index.md
@@ -461,6 +461,7 @@ CDash Dashboard <https://open.cdash.org/index.php?project=Insight>
 dashboard.md
 updating_third_party.md
 python_packaging.md
+wrapping_architecture.md
 ../README.md
 ```
 

diff --git a/Documentation/docs/contributing/wrapping_architecture.md b/Documentation/docs/contributing/wrapping_architecture.md
@@ -0,0 +1,215 @@
+Python Wrapping Architecture
+============================
+
+ITK's Python wrapping pipeline converts C++ template declarations into
+importable Python extension modules (`.abi3.so`) and type-stub files
+(`.pyi`). The pipeline runs in two distinct phases: a **configure phase**
+driven by CMake and a **build phase** driven by Ninja.
+
+## Configure phase: `.wrap` → `castxml_inputs/`
+
+Each ITK module that supports wrapping contains a `wrapping/` subdirectory
+with one `.wrap` file per submodule. A `.wrap` file is a CMake script that
+calls macros such as `itk_wrap_class()` and `itk_wrap_template()` to declare
+which C++ template instantiations should be exposed to Python.
+
+CMake processes every `.wrap` file at configure time and writes three
+files per submodule into `<build>/Wrapping/castxml_inputs/`:
+
+| Generated file | How | Contents |
+|---|---|---|
+| `<submodule>.cxx` | `configure_file` | `#include` directives + `_wrapping_` namespace with `using` aliases for every requested template instantiation |
+| `<submodule>.castxml.inc` | `file(GENERATE …)` | Compiler `-I` and `-D` flags needed to parse the `.cxx` file |
+| `<submodule>SwigInterface.h.in` | `configure_file` | `#include` list used by SWIG |
+
+CMake also registers one `add_custom_command` per submodule (for CastXML)
+and one per ITK module (for `igenerator.py`). No compilation happens at
+configure time; only the input files and build rules are written.
+
+## Build phase: CastXML → igenerator → SWIG → compile → link
+
+### Step 1 — CastXML (816 independent jobs)
+
+Each submodule produces exactly one XML file:
+
+```
+castxml_inputs/<submodule>.cxx
+castxml_inputs/<submodule>.castxml.inc     ──▶  itk-castxml-cache.py  ──▶  castxml_inputs/<submodule>.xml
+Modules/.../include/<Class>.h (many)
+```
+
+`itk-castxml-cache.py` wraps the CastXML binary with a two-level
+content-addressed cache (`~/.cache/itk-wrap` or `$ITK_WRAP_CACHE`):
+
+- **L1** — hash of the `.cxx` file content → L2 key
+- **L2** — `castxml -E` (preprocessor only) output hash → cached `output.xml.gz`
+
+A cache hit avoids running CastXML entirely. All 816 CastXML jobs are
+independent and run fully in parallel. No CastXML job reads or modifies
+another submodule's `.xml` output.
+
+### Step 2 — `igenerator.py` (96 per-module jobs)
+
+Each ITK module (e.g. `ITKImageIntensity`) batches all of its submodules
+into a single `igenerator.py` invocation:
+
+```
+castxml_inputs/itkAbsImageFilter.xml ──┐
+castxml_inputs/itkImage.xml          ──┤
+castxml_inputs/itkOffset.xml         ──┤  igenerator.py [ITKImageIntensity]
+... (all N submodule XMLs)           ──┘  --submodule-order "sub1;sub2;...;subN"
+                                               │
+                    ┌──────────────────────────┼─────────────────────────────┐
+                    │  per submodule (×N)       │                             │
+                    ▼                           ▼                             ▼
+         Typedefs/<sub>.i            itk-pkl/<sub>.index.txt      itk-pkl-v3.db  (SQLite)
+         Typedefs/<sub>.idx          (lists DB keys; byproduct)   (class metadata; WAL mode)
+         Typedefs/<sub>SwigInterface.h
+                                          + per module (×1):
+                                            itk-pkl/<Module>.stamp
+                                            itk/Configuration/<Module>_snake_case.py
+```
+
+`igenerator.py` uses `pygccxml` to parse each `.xml` file and emit SWIG
+interface (`.i`) and index (`.idx`) files, class-metadata rows in a
+build-local SQLite database consumed later by `pyi_generator.py`, and a
+`.index.txt` manifest listing the DB keys for each submodule.
+
+The SQLite database (`itk-pkl-v3.db`) is written to the `itk-pkl/` directory
+inside the build tree.  It is intermediate handoff state keyed by bare class
+name, so it is always build-tree-local and is never shared via
+`ITK_WRAP_CACHE`.
+
+**Ninja scheduling**: an `igenerator.py` job for module A starts as soon
+as all of A's CastXML jobs are complete, even while CastXML is still
+running for module B. There is no global barrier between the CastXML and
+`igenerator.py` layers.
+
+### Step 3 — SWIG, compile, link (per submodule)
+
+```
+Typedefs/<sub>.i
+Typedefs/<sub>SwigInterface.h  ──▶  swig  ──▶  Modules/.../<sub>Python.cpp
+                                                Generators/Python/itk/<sub>Python.py
+
+Modules/.../<sub>Python.cpp  ──▶  ccache + g++  ──▶  .o  ──▶  link  ──▶  _<Module>Python.abi3.so
+```
+
+### Step 4 — `pyi_generator.py` (one global job)
+
+After **all** 816 `.index.txt` files exist, `pyi_generator.py` reads every
+`.index.txt`, queries the SQLite database for each key, and writes the
+type-stub files used by IDEs:
+
+```
+itk-pkl/<sub>.index.txt (×816) ──▶  pyi_generator.py  ──▶  _proxies.pyi
+itk-pkl-v3.db (SQLite)               (queries DB via keys in .index.txt)  __init__.pyi
+```
+
+## Key file reference
+
+| Path pattern | Written by | Read by |
+|---|---|---|
+| `Wrapping/castxml_inputs/<sub>.cxx` | CMake `configure_file` | CastXML |
+| `Wrapping/castxml_inputs/<sub>.castxml.inc` | CMake `file(GENERATE)` | `itk-castxml-cache.py` |
+| `Wrapping/castxml_inputs/<sub>.xml` | `itk-castxml-cache.py` / CastXML | `igenerator.py` |
+| `Wrapping/Typedefs/<sub>.i` | `igenerator.py` | SWIG |
+| `Wrapping/Typedefs/<sub>.idx` | `igenerator.py` | SWIG |
+| `Wrapping/Generators/Python/itk-pkl/<sub>.index.txt` | `igenerator.py` | `pyi_generator.py` |
+| `Wrapping/Generators/Python/itk-pkl/itk-pkl-v3.db` | `igenerator.py` | `pyi_generator.py` |
+| `Wrapping/Generators/Python/itk-pkl/<Module>.stamp` | `igenerator.py` | ninja (tracks DB write completeness) |
+| `Wrapping/Generators/Python/itk/_<Module>Python.abi3.so` | linker | Python `import itk` |
+| `Wrapping/Generators/Python/itk/_proxies.pyi` | `pyi_generator.py` | IDEs |
+
+## Caches
+
+### CastXML cache (`itk-castxml-cache.py`)
+
+| Variable | Default | Purpose |
+|---|---|---|
+| `ITK_WRAP_CACHE` | `~/.cache/itk-wrap` | Cache root for CastXML `.xml.gz` files |
+| `ITK_WRAP_CACHE_VERBOSE` | unset | Set to `1` to log HIT/MISS per file |
+
+The CastXML cache is content-addressed and generator-version-stamped
+(`_KEY_VERSION` in `itk-castxml-cache.py`). It is shared across build
+directories; a fresh configure reuses XML from a previous build on the same
+machine.
+
+### pkl SQLite database (`igenerator.py` / `pyi_generator.py`)
+
+The pkl database (`itk-pkl-v3.db`) lives in the build tree's `itk-pkl/`
+directory and is a build artifact, not a user-level cache.  Unlike the
+content-addressed CastXML cache, it is keyed by bare `class.submodule`
+strings, so it is **always build-tree-local and never honors
+`ITK_WRAP_CACHE`** — sharing it across build trees would collide same-named
+classes from different ITK sources.
+
+`igenerator.py` writes the rows; `pyi_generator.py` reads them and prunes any
+row whose key is absent from the current build's `.index.txt` manifests
+(exact keyset cleanup).  The schema version in the filename means a database
+from an older schema is ignored rather than migrated.
+
+### ccache
+
+CastXML re-runs produce identical `.xml` files (the content is deterministic)
+but are slow. The CastXML cache eliminates that cost. For the C++ compilation
+steps (Step 3), ccache caches compiled `.o` files keyed on source content.
+Both caches are independent and complement each other.
+
+### Build-phase timing on 2-core CI runners
+
+On a 2-core CI runner (cold caches throughout):
+
+| Phase | Approx. time | Notes |
+|---|---|---|
+| CastXML (816 jobs, 2 cores) | ~32 min | Eliminated on warm-cache runs |
+| igenerator + SWIG + C++ compile | ~225 min | Reduced by ccache on subsequent runs |
+| Tests | ~44 min | |
+
+CastXML is ~10 % of the cold-cache total.  The C++ compilation phase
+dominates; `ccache` is the primary lever there.
+
+## Ninja dependency graph summary
+
+```
+.wrap files (configure time, not in graph)
+    │ cmake configure_file / file(GENERATE)
+    ▼
+castxml_inputs/<sub>.cxx + .castxml.inc + .h headers
+    │ itk-castxml-cache.py  [816 parallel jobs]
+    ▼
+castxml_inputs/<sub>.xml          (write-once; never mutated after creation)
+    │ igenerator.py  [96 per-module jobs; starts per-module, not globally gated]
+    ▼
+Typedefs/<sub>.i + .idx + SwigInterface.h
+itk-pkl/<sub>.index.txt (DB keys) + itk-pkl-v3.db (SQLite) + <Module>.stamp
+    │ swig + ccache compile + link  [parallel per submodule]
+    ▼
+_<Module>Python.abi3.so
+    │ pyi_generator.py  [1 global job; needs all .index.txt]
+    ▼
+_proxies.pyi + __init__.pyi
+```
+
+## Troubleshooting
+
+**CastXML cache not being used**
+: Set `ITK_WRAP_CACHE_VERBOSE=1` and rebuild one module to confirm HIT or
+  MISS log lines. Ensure `ITK_WRAP_CASTXML_CACHE=ON` is set in CMake.
+  A version bump to `_KEY_VERSION` in `itk-castxml-cache.py` forces a cold
+  cache for all entries.
+
+**`No pkl keys were found in index files in itk-pkl`**
+: The `.index.txt` manifests exist (so ninja considers `igenerator.py`
+  up-to-date) but the pkl database is absent or stale. Delete the stamp
+  files to force `igenerator.py` to re-run and repopulate the DB:
+  ```bash
+  find <build>/Wrapping/Generators/Python/itk-pkl -name "*.index.txt" -o -name "*.stamp" | xargs rm -f
+  ninja -C <build>
+  ```
+
+**Adding a new wrapped class**
+: Add `itk_wrap_class()` / `itk_wrap_template()` calls to the relevant
+  `.wrap` file. Re-run CMake to regenerate the `.cxx` and `.castxml.inc`
+  files, then build normally. CMake will automatically include the new
+  submodule in the `--submodule-order` passed to `igenerator.py`.
diff --git a/Testing/ContinuousIntegration/AzurePipelinesLinuxPython.yml b/Testing/ContinuousIntegration/AzurePipelinesLinuxPython.yml
@@ -36,6 +36,7 @@ variables:
   CCACHE_NOHASHDIR: 'true'
   CCACHE_SLOPPINESS: pch_defines,time_macros
   CCACHE_MAXSIZE: 8G
+  ITK_WRAP_CACHE: $(Pipeline.Workspace)/.castxml-cache
 jobs:
 - job: Linux
   timeoutInMinutes: 0
@@ -55,6 +56,7 @@ jobs:
         df -h /
       displayName: 'Free preinstalled software'
 
+
     - checkout: self
       clean: true
       fetchDepth: 5
@@ -102,6 +104,14 @@ jobs:
         path: $(CCACHE_DIR)
       displayName: 'Restore ccache'
 
+    - task: Cache@2
+      inputs:
+        key: '"castxml-v1" | "$(Agent.OS)" | "LinuxPython" | "$(Build.SourceVersion)"'
+        restoreKeys: |
+          "castxml-v1" | "$(Agent.OS)" | "LinuxPython"
+        path: $(ITK_WRAP_CACHE)
+      displayName: 'Restore CastXML cache'
+
     - bash: |
         set -x
         ccache --zero-stats
@@ -123,6 +133,7 @@ jobs:
           BUILD_SHARED_LIBS:BOOL=OFF
           BUILD_EXAMPLES:BOOL=OFF
           ITK_WRAP_PYTHON:BOOL=ON
+          ITK_WRAP_CASTXML_CACHE:BOOL=ON
           CMAKE_C_COMPILER_LAUNCHER:STRING=ccache
           CMAKE_CXX_COMPILER_LAUNCHER:STRING=ccache
           ITK_COMPUTER_MEMORY_SIZE:STRING=4.5

diff --git a/Testing/ContinuousIntegration/AzurePipelinesMacOSPython.yml b/Testing/ContinuousIntegration/AzurePipelinesMacOSPython.yml
@@ -36,6 +36,7 @@ variables:
   CCACHE_NOHASHDIR: 'true'
   CCACHE_SLOPPINESS: pch_defines,time_macros
   CCACHE_MAXSIZE: 8G
+  ITK_WRAP_CACHE: $(Pipeline.Workspace)/.castxml-cache
 jobs:
 - job: macOS
   timeoutInMinutes: 0
@@ -106,6 +107,14 @@ jobs:
         path: $(CCACHE_DIR)
       displayName: 'Restore ccache'
 
+    - task: Cache@2
+      inputs:
+        key: '"castxml-v1" | "$(Agent.OS)" | "macOSPython" | "$(Build.SourceVersion)"'
+        restoreKeys: |
+          "castxml-v1" | "$(Agent.OS)" | "macOSPython"
+        path: $(ITK_WRAP_CACHE)
+      displayName: 'Restore CastXML cache'
+
     - bash: |
         set -x
         ccache --zero-stats
@@ -127,6 +136,7 @@ jobs:
           BUILD_SHARED_LIBS:BOOL=ON
           BUILD_EXAMPLES:BOOL=OFF
           ITK_WRAP_PYTHON:BOOL=ON
+          ITK_WRAP_CASTXML_CACHE:BOOL=ON
           CMAKE_C_COMPILER_LAUNCHER:STRING=ccache
           CMAKE_CXX_COMPILER_LAUNCHER:STRING=ccache
           ITK_COMPUTER_MEMORY_SIZE:STRING=11