diff --git a/.circleci/config.yml b/.circleci/config.yml
index 27c16c14c3..263a46c424 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -1,13 +1,13 @@
 version: 2.1
 
 orbs:
-  python: circleci/python@2.1.1
+  python: circleci/python@3.0.0
 
 jobs:
   manylinux2014-aarch64:
 
     parameters:
-      NRN_PYTHON_VERSION:
+      NRN_PYTHON_VERSION_MINOR:
         type: string
       NRN_NIGHTLY_UPLOAD:
         type: string
@@ -31,8 +31,8 @@ jobs:
                 -e NRN_RELEASE_UPLOAD \
                 -e SETUPTOOLS_SCM_PRETEND_VERSION \
                 -e NRN_BUILD_FOR_UPLOAD=1 \
-                'neuronsimulator/neuron_wheel:latest-gcc9-aarch64' \
-                packaging/python/build_wheels.bash linux << parameters.NRN_PYTHON_VERSION >> coreneuron
+                'neuronsimulator/neuron_wheel:latest-aarch64' \
+                packaging/python/build_wheels.bash linux 3<< parameters.NRN_PYTHON_VERSION_MINOR >> coreneuron
 
       - store_artifacts:
           path: ./wheelhouse
@@ -41,24 +41,16 @@ jobs:
       - run:
           name: Test manylinux AArch64 wheel
           command: |
-
-            # install mpi dependencies
             sudo apt update
+            echo "deb http://ppa.launchpad.net/deadsnakes/ppa/ubuntu jammy main" | sudo tee /etc/apt/sources.list.d/deadsnakes-ppa.list && sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys F23C5A6CF475977595C89F51BA6932366A755776 && sudo apt update
+            sudo apt update
+            # install mpi dependencies
             sudo apt install -y mpich openmpi-bin libopenmpi-dev libmpich-dev
+            version=3.<< parameters.NRN_PYTHON_VERSION_MINOR >>
+            # install Python from deadsnakes
+            sudo apt install -y python${version}-venv libpython${version}-dev g++ make
 
-            # choose available python versions from pyenv
-            pyenv_py_ver=""
-            case << parameters.NRN_PYTHON_VERSION >> in
-              39) pyenv_py_ver="3.9" ;;
-              310) pyenv_py_ver="3.10" ;;
-              311) pyenv_py_ver="3.11" ;;
-              312) pyenv_py_ver="3.12" ;;
-              *) echo "Error: pyenv python version not specified or not supported." && exit 1;;
-            esac
-
-            env PYTHON_CONFIGURE_OPTS="--enable-shared" pyenv install $pyenv_py_ver --force
-            pyenv global $pyenv_py_ver
-            export PYTHON_EXE=$(which python)
+            export PYTHON_EXE=$(which python3.<< parameters.NRN_PYTHON_VERSION_MINOR >>)
 
             # test wheel
             packaging/python/test_wheels.sh $PYTHON_EXE $(ls -t wheelhouse/*.whl)
@@ -86,7 +78,7 @@ workflows:
                 - /circleci\/.*/
           matrix:
             parameters:
-              NRN_PYTHON_VERSION: ["312"]
+              NRN_PYTHON_VERSION_MINOR: ["13"]
               NRN_NIGHTLY_UPLOAD: ["false"]
 
   nightly:
@@ -101,5 +93,5 @@ workflows:
       - manylinux2014-aarch64:
           matrix:
             parameters:
-              NRN_PYTHON_VERSION: ["39", "310", "311", "312"]
+              NRN_PYTHON_VERSION_MINOR: ["9", "10", "11", "12", "13"]
               NRN_NIGHTLY_UPLOAD: ["true"]
diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
index a4e23da01c..f204e36421 100644
--- a/.github/workflows/coverage.yml
+++ b/.github/workflows/coverage.yml
@@ -24,7 +24,7 @@ on:
 env:
   PY_MIN_VERSION: '3.9'
   PY_MID_VERSION: '3.10'
-  PY_MAX_VERSION: '3.12'
+  PY_MAX_VERSION: '3.13'
 
 jobs:
   coverage:
diff --git a/.github/workflows/docs.yml b/.github/workflows/docs.yml
index 2ef1052dd7..f9cc6e0051 100644
--- a/.github/workflows/docs.yml
+++ b/.github/workflows/docs.yml
@@ -17,7 +17,7 @@ on:
       - release/**
 
 env:
-  DEFAULT_PY_VERSION: '3.12'
+  DEFAULT_PY_VERSION: '3.13'
 
 jobs:
   documentation:
diff --git a/.github/workflows/neuron-ci.yml b/.github/workflows/neuron-ci.yml
index 48ec76d28e..adcf08c3b7 100644
--- a/.github/workflows/neuron-ci.yml
+++ b/.github/workflows/neuron-ci.yml
@@ -39,7 +39,7 @@ jobs:
       DESIRED_CMAKE_VERSION: 3.17
       DYNAMIC_PYTHON_CMAKE_VERSION: 3.18
       PY_MIN_VERSION: ${{ matrix.config.python_min_version || '3.9' }}
-      PY_MAX_VERSION: ${{ matrix.config.python_max_version || '3.12' }}
+      PY_MAX_VERSION: ${{ matrix.config.python_max_version || '3.13' }}
       MUSIC_INSTALL_DIR: /opt/MUSIC
       # hash of commit containing mpi4py 4 fix
       MUSIC_VERSION: '13f312338dcccebfe74d391b1b24f1b6d816ac6c'
diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index 02f5f8374a..5970a4243f 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -42,6 +42,9 @@ stages:
             python.version: '3.11'
           Python312:
             python.version: '3.12'
+          Python313:
+            python.version: '3.13'
+
       steps:
 
       # Secure files documentation:
@@ -74,7 +77,7 @@ stages:
       - script: |
           sudo apt update
           sudo apt install -y mpich openmpi-bin libopenmpi-dev libmpich-dev
-        displayName: 'Install Test System Depdendencies'
+        displayName: 'Install Test System Dependencies'
 
       - template: ci/azure-wheel-test-upload.yml
 
@@ -102,6 +105,10 @@ stages:
             python.version: '3.12'
             python.org.version: '3.12.0'
             python.installer.name: 'macos11.pkg'
+          Python313:
+            python.version: '3.13'
+            python.org.version: '3.13.0'
+            python.installer.name: 'macos11.pkg'
 
       steps:
 
diff --git a/bldnrnmacpkgcmake.sh b/bldnrnmacpkg.sh
similarity index 76%
rename from bldnrnmacpkgcmake.sh
rename to bldnrnmacpkg.sh
index 2f0a56af8d..49ed663691 100644
--- a/bldnrnmacpkgcmake.sh
+++ b/bldnrnmacpkg.sh
@@ -1,19 +1,29 @@
 #!/usr/bin/env bash
 set -ex
 
-default_pythons="python3.8 python3.9 python3.10 python3.11"
+default_pythons="python3.9 python3.10 python3.11 python3.12 python3.13"
 # distribution built with
 # bash bldnrnmacpkgcmake.sh
 # without args, default are the pythons above.
 
+if test "$RX3D_OPT" = "" ; then
+  RX3D_OPT=1 # a value of 2 takes much longer to build
+fi
+
 # If all the pythons are universal, then so is NEURON.
 # Otherwise $CPU
-# All pythons must have the same macos version and that will become
-# the MACOSX_DEPLOYMENT_TARGET
 
+# Now obsolete...
 # On my machine, to build nrn-x.x.x-macosx-10.9-universal2-py-38-39-310-311.pkg
 # I built my own versions of 3.8 in $HOME/soft/python3.8, and
-export PATH=$HOME/soft/python3.8/bin:$PATH
+# export PATH=$HOME/soft/python3.8/bin:$PATH
+# All other python versions installed from python.org universal2 installers.
+
+# As of Python-3.13.0, we configure to link universal2 static readline
+# (and ncurses) into libnrniv.dylib.
+# The packages were downloade and the universal libraries were built with
+# nrn/packaging/python/build_static_readline_osx.bash
+READLINE_LIST="/opt/nrnwheel/arm64/readline;/opt/nrnwheel/arm64/ncurses"
 
 CPU=`uname -m`
 
@@ -24,10 +34,15 @@ if test "$args" = "" ; then
   args="$default_pythons"
 fi
 
+MPI_LIST="/opt/homebrew/opt/openmpi/include;/opt/homebrew/opt/mpich/include"
 
 # sysconfig.get_platform() looks like, e.g. "macosx-12.2-arm64" or
 # "macosx-11-universal2". I.e. encodes MACOSX_DEPLOYMENT_TARGET and archs.
 # Demand all pythons we are building against have same platform.
+# Update: nrn software now requires at least 10.15. All the python's on this
+# machine are 10.9, except python 3.13 is 10.13. The substantive aspect of
+# the following fragment (exit 1 if not all the same platform), has been
+# commented out.
 mac_platform=""
 for i in $args ; do
   last_py=$i
@@ -38,7 +53,7 @@ for i in $args ; do
   fi
   if test "$mac_platform" != "$mplat" ; then
     echo "$i platform \"$mplat\" differs from previous python \"$mac_platform\"."
-    exit 1
+#    exit 1
   fi
 done
 
@@ -93,13 +108,15 @@ fi
 
 cmake .. -G Ninja -DCMAKE_INSTALL_PREFIX=$NRN_INSTALL \
   -DNRN_ENABLE_MPI_DYNAMIC=ON \
+  -DNRN_MPI_DYNAMIC="$MPI_LIST" \
   -DPYTHON_EXECUTABLE=`which python3` -DNRN_ENABLE_PYTHON_DYNAMIC=ON \
   -DNRN_PYTHON_DYNAMIC="$pythons" \
   -DIV_ENABLE_X11_DYNAMIC=ON \
   -DNRN_ENABLE_CORENEURON=OFF \
-  -DNRN_RX3D_OPT_LEVEL=2 \
+  -DNRN_RX3D_OPT_LEVEL=$RX3D_OPT \
+  -DNRN_WHEEL_STATIC_READLINE=ON \
   $archs_cmake \
-  -DCMAKE_PREFIX_PATH=/usr/X11 \
+  -DCMAKE_PREFIX_PATH="/usr/X11;$READLINE_LIST" \
   -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++
 
 ninja install
@@ -134,22 +151,29 @@ for i in $args ; do
   chk $i
 done
 
+describe="`sh $NRN_SRC/nrnversion.sh describe`"
+macos=macos${MACOSX_DEPLOYMENT_TARGET}
+PACKAGE_FULL_NAME=nrn-${describe}-${mac_platform}-${PYVS}.pkg
+PACKAGE_FILE_NAME=$NRN_BLD/src/mac/build/NEURON.pkg
+
 #/Applications/Packages.app from
 # http://s.sudre.free.fr/Software/Packages/about.html
 # For mac to do a productsign, need my developerID_installer.cer
 # and Neurondev.p12 file. To add to the keychain, double click each
 # of those files. By default, I added my certificates to the login keychain.
-ninja macpkg # will sign the binaries, construct below
-            # mentioned PACKAGE_FILE_NAME, request notarization from
-            # Apple, and staple the package.
+
+# Will sign the binaries, construct above
+# mentioned PACKAGE_FILE_NAME, request notarization from
+# Apple, and staple the package.
+if ! ninja macpkg ; then 
+  echo "ninja macpkg failed. (because of notification failure?)"
+  echo "Not creating $PACKAGE_FULL_NAME"
+  echo "  from $PACKAGE_FILE_NAME"
+  exit 1
+fi
 
 # Copy the package to $HOME/$PACKAGE_FULL_NAME
 # You should then manually upload that to github.
-describe="`sh $NRN_SRC/nrnversion.sh describe`"
-macos=macos${MACOSX_DEPLOYMENT_TARGET}
-PACKAGE_FULL_NAME=nrn-${describe}-${mac_platform}-${PYVS}.pkg
-PACKAGE_FILE_NAME=$NRN_BLD/src/mac/build/NEURON.pkg
-
 cp $PACKAGE_FILE_NAME $HOME/$PACKAGE_FULL_NAME
 
 echo "
diff --git a/ci/win_build_cmake.sh b/ci/win_build_cmake.sh
index b82e430444..15cc217323 100755
--- a/ci/win_build_cmake.sh
+++ b/ci/win_build_cmake.sh
@@ -32,7 +32,7 @@ cd $BUILD_SOURCESDIRECTORY/build
 	-DNRN_BINARY_DIST_BUILD=ON \
 	-DPYTHON_EXECUTABLE=/c/Python39/python.exe \
 	-DNRN_ENABLE_PYTHON_DYNAMIC=ON  \
-	-DNRN_PYTHON_DYNAMIC='c:/Python39/python.exe;c:/Python310/python.exe;c:/Python311/python.exe;c:/Python312/python.exe'  \
+	-DNRN_PYTHON_DYNAMIC='c:/Python39/python.exe;c:/Python310/python.exe;c:/Python311/python.exe;c:/Python312/python.exe;c:/Python313/python.exe'  \
 	-DCMAKE_INSTALL_PREFIX='/c/nrn-install' \
 	-DMPI_CXX_LIB_NAMES:STRING=msmpi \
 	-DMPI_C_LIB_NAMES:STRING=msmpi \
diff --git a/ci/win_download_deps.cmd b/ci/win_download_deps.cmd
index 40a9b44f49..166d44461a 100644
--- a/ci/win_download_deps.cmd
+++ b/ci/win_download_deps.cmd
@@ -7,6 +7,7 @@ pwsh -command Invoke-WebRequest -MaximumRetryCount 4 -OutFile python-3.9.exe htt
 pwsh -command Invoke-WebRequest -MaximumRetryCount 4 -OutFile python-3.10.exe https://www.python.org/ftp/python/3.10.0/python-3.10.0-amd64.exe || goto :error
 pwsh -command Invoke-WebRequest -MaximumRetryCount 4 -OutFile python-3.11.exe https://www.python.org/ftp/python/3.11.1/python-3.11.1-amd64.exe || goto :error
 pwsh -command Invoke-WebRequest -MaximumRetryCount 4 -OutFile python-3.12.exe https://www.python.org/ftp/python/3.12.1/python-3.12.1-amd64.exe || goto :error
+pwsh -command Invoke-WebRequest -MaximumRetryCount 4 -OutFile python-3.13.exe https://www.python.org/ftp/python/3.13.0/python-3.13.0-amd64.exe || goto :error
 
 :: mpi
 pwsh -command Invoke-WebRequest -MaximumRetryCount 4 -OutFile msmpisetup.exe https://download.microsoft.com/download/a/5/2/a5207ca5-1203-491a-8fb8-906fd68ae623/msmpisetup.exe || goto :error
diff --git a/ci/win_install_deps.cmd b/ci/win_install_deps.cmd
index c12f32cdef..82a8c950d1 100644
--- a/ci/win_install_deps.cmd
+++ b/ci/win_install_deps.cmd
@@ -7,6 +7,7 @@ python-3.9.exe /passive Include_pip=1 Include_test=0 PrependPath=1 DefaultJustFo
 python-3.10.exe /passive Include_pip=1 Include_test=0 PrependPath=1 DefaultJustForMeTargetDir=C:\Python310 || goto :error
 python-3.11.exe /passive Include_pip=1 Include_test=0 PrependPath=1 DefaultJustForMeTargetDir=C:\Python311 || goto :error
 python-3.12.exe /passive Include_pip=1 Include_test=0 PrependPath=1 DefaultJustForMeTargetDir=C:\Python312 || goto :error
+python-3.13.exe /passive Include_pip=1 Include_test=0 PrependPath=1 DefaultJustForMeTargetDir=C:\Python313 || goto :error
 
 :: fix msvcc version for all python3
 pwsh -command "(Get-Content C:\Python39\Lib\distutils\cygwinccompiler.py) -replace 'elif msc_ver == ''1600'':', 'elif msc_ver == ''1927'':' | Out-File C:\Python39\Lib\distutils\cygwinccompiler.py"
@@ -24,8 +25,10 @@ C:\Python39\python.exe -m pip install  numpy cython || goto :error
 C:\Python310\python.exe -m pip install numpy cython || goto :error
 C:\Python311\python.exe -m pip install numpy cython || goto :error
 C:\Python312\python.exe -m pip install numpy cython || goto :error
+C:\Python313\python.exe -m pip install numpy cython || goto :error
 :: setuptools 70.2 leads to an error
 C:\Python312\python.exe -m pip install setuptools==70.1.1 || goto :error
+C:\Python313\python.exe -m pip install setuptools==70.1.1 || goto :error
 
 :: install nsis
 nsis-3.05-setup.exe /S || goto :error
diff --git a/ci/win_test_installer.cmd b/ci/win_test_installer.cmd
index bda5bbec7b..a6cf59a009 100644
--- a/ci/win_test_installer.cmd
+++ b/ci/win_test_installer.cmd
@@ -21,18 +21,21 @@ C:\Python39\python -c "import neuron; neuron.test(); quit()" || set "errorfound=
 C:\Python310\python -c "import neuron; neuron.test(); quit()" || set "errorfound=y"
 C:\Python311\python -c "import neuron; neuron.test(); quit()" || set "errorfound=y"
 C:\Python312\python -c "import neuron; neuron.test(); quit()" || set "errorfound=y"
+C:\Python313\python -c "import neuron; neuron.test(); quit()" || set "errorfound=y"
 
 :: install oldest supported numpy
 C:\Python39\python.exe -m pip install -r packaging/python/oldest_numpy_requirements.txt || goto :error
 C:\Python310\python.exe -m pip install -r packaging/python/oldest_numpy_requirements.txt || goto :error
 C:\Python311\python.exe -m pip install -r packaging/python/oldest_numpy_requirements.txt || goto :error
 C:\Python312\python.exe -m pip install -r packaging/python/oldest_numpy_requirements.txt || goto :error
+C:\Python313\python.exe -m pip install -r packaging/python/oldest_numpy_requirements.txt || goto :error
 
 :: test all pythons again
 C:\Python39\python -c "import neuron; neuron.test(); quit()" || set "errorfound=y"
 C:\Python310\python -c "import neuron; neuron.test(); quit()" || set "errorfound=y"
 C:\Python311\python -c "import neuron; neuron.test(); quit()" || set "errorfound=y"
 C:\Python312\python -c "import neuron; neuron.test(); quit()" || set "errorfound=y"
+C:\Python313\python -c "import neuron; neuron.test(); quit()" || set "errorfound=y"
 
 :: run also using whatever is system python
 python -m pip install numpy
diff --git a/docs/python/modelspec/programmatic/topology/seclist.rst b/docs/python/modelspec/programmatic/topology/seclist.rst
index 3ff618c188..810d145d62 100755
--- a/docs/python/modelspec/programmatic/topology/seclist.rst
+++ b/docs/python/modelspec/programmatic/topology/seclist.rst
@@ -31,6 +31,10 @@ SectionList
             for sec in python_iterable_of_sections:
                 sl.append(sec)
 
+        ``len(sl)`` returns the number of sections in the SectionList.
+
+        ``list(sl)`` and ``[s for s in sl]`` generate equivalent lists.
+
     .. seealso::
         :class:`SectionBrowser`, :class:`Shape`, :meth:`RangeVarPlot.list`
 
diff --git a/packaging/python/Dockerfile b/packaging/python/Dockerfile
index 36c7d0ad4f..95931335cc 100644
--- a/packaging/python/Dockerfile
+++ b/packaging/python/Dockerfile
@@ -93,4 +93,7 @@ COPY Dockerfile .
 # build wheels from there
 WORKDIR /root
 
+# remove Python 3.13t since we do not support the free-threaded build yet
+RUN rm -fr /opt/python/cp313-cp313t
+
 ENV NMODL_PYLIB=/nrnwheel/python/lib/libpython3.10.so.1.0
diff --git a/packaging/python/oldest_numpy_requirements.txt b/packaging/python/oldest_numpy_requirements.txt
index 4a77e64c2b..eb2e1a80db 100644
--- a/packaging/python/oldest_numpy_requirements.txt
+++ b/packaging/python/oldest_numpy_requirements.txt
@@ -3,3 +3,4 @@ numpy==1.21.6;python_version=='3.9' and platform_machine=='arm64'
 numpy==1.21.6;python_version=='3.10'
 numpy==1.23.5;python_version=='3.11'
 numpy==1.26.4;python_version=='3.12'
+numpy==2.1.0;python_version=='3.13'
diff --git a/setup.py b/setup.py
index 05e8ca67e1..25c3fabc2a 100644
--- a/setup.py
+++ b/setup.py
@@ -4,6 +4,7 @@
 import sys
 from collections import defaultdict
 import logging
+import platform
 
 logging.basicConfig(level=logging.INFO)
 from shutil import copytree, which
@@ -461,6 +462,8 @@ def setup_package():
                 + ["-Wl,-rpath,{}".format(REL_RPATH + "/../../.data/lib/")],
             )
         )
+        if platform.system() == "Darwin":
+            rxd_params["extra_link_args"] += ["-headerpad_max_install_names"]
 
         logging.info("RX3D compile flags %s" % str(rxd_params))
 
diff --git a/src/coreneuron/io/nrn_setup.cpp b/src/coreneuron/io/nrn_setup.cpp
index 3fe338348f..8cdd33bee2 100644
--- a/src/coreneuron/io/nrn_setup.cpp
+++ b/src/coreneuron/io/nrn_setup.cpp
@@ -220,6 +220,12 @@ std::vector<int> nrn_read_filesdat(const char* filesdat) {
         int iFile;
         nrn_assert(fscanf(fp, "%d\n", &iFile) == 1);
         if ((iNum % nrnmpi_numprocs) == nrnmpi_myid) {
+            // A "-1" entry means that this rank should not be assigned further gid groups.
+            // It is a way to create files.dat files which deterministically assign gid groups to
+            // ranks, particularly useful for very large simulations which required load balancing.
+            if (iFile == -1) {
+                break;
+            }
             rank_cell_groups.push_back(iFile);
         }
     }
diff --git a/src/mac/nrn_productsign.sh b/src/mac/nrn_productsign.sh
index cbe1d78122..84007ad2ee 100755
--- a/src/mac/nrn_productsign.sh
+++ b/src/mac/nrn_productsign.sh
@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+set -ex
 
 # Signing the package with an identified developer certificate means the
 # installer does not have to lower the security settings on their machine.
diff --git a/src/nrniv/secbrows.cpp b/src/nrniv/secbrows.cpp
index 84263229b6..207a5b6b4f 100644
--- a/src/nrniv/secbrows.cpp
+++ b/src/nrniv/secbrows.cpp
@@ -121,17 +121,13 @@ OcSectionBrowser::OcSectionBrowser(Object* ob)
             psec_[scnt_++] = sec;
         }
     } else {
-        struct hoc_Item* qsec;
         scnt_ = 0;
-        // ForAllSections(sec)  //{
-        ITERATE(qsec, section_list) {
+        for (const Section* sec: range_sec(section_list)) {
             ++scnt_;
         }
         psec_ = new Section*[scnt_];
         scnt_ = 0;
-        // ForAllSections(sec)  //{
-        ITERATE(qsec, section_list) {
-            Section* sec = hocSEC(qsec);
+        for (Section* sec: range_sec(section_list)) {
             psec_[scnt_++] = sec;
         }
     }
@@ -359,18 +355,13 @@ void BrowserAccept::execute() {
 }
 
 SectionBrowserImpl::SectionBrowserImpl() {
-    struct hoc_Item* qsec;
     scnt_ = 0;
-    // ForAllSections(sec)  //{
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (const Section* sec: range_sec(section_list)) {
         ++scnt_;
     }
     psec_ = new Section*[scnt_];
     scnt_ = 0;
-    // ForAllSections(sec)  //{
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (Section* sec: range_sec(section_list)) {
         psec_[scnt_++] = sec;
         section_ref(sec);
     }
diff --git a/src/nrnoc/cabcode.cpp b/src/nrnoc/cabcode.cpp
index fe6de4e3a2..85c4b1b392 100644
--- a/src/nrnoc/cabcode.cpp
+++ b/src/nrnoc/cabcode.cpp
@@ -450,10 +450,7 @@ Section* chk_access() {
     Section* sec = secstack[isecstack];
     if (!sec || !sec->prop) {
         /* use any existing section as a default section */
-        hoc_Item* qsec;
-        // ForAllSections(lsec)
-        ITERATE(qsec, section_list) {
-            Section* lsec = hocSEC(qsec);
+        for (Section* lsec: range_sec(section_list)) {
             if (lsec->prop) {
                 sec = lsec;
                 ++sec->refcount;
@@ -479,10 +476,7 @@ Section* nrn_noerr_access(void) /* return 0 if no accessed section */
     Section* sec = secstack[isecstack];
     if (!sec || !sec->prop) {
         /* use any existing section as a default section */
-        hoc_Item* qsec;
-        // ForAllSections(lsec)
-        ITERATE(qsec, section_list) {
-            Section* lsec = hocSEC(qsec);
+        for (Section* lsec: range_sec(section_list)) {
             if (lsec->prop) {
                 sec = lsec;
                 ++sec->refcount;
@@ -1639,8 +1633,6 @@ void nrn_parent_info(Section* s) {
 }
 
 void setup_topology(void) {
-    Item* qsec;
-
     /* use connection info in section property to connect nodes. */
     /* for the moment we assume uniform dx and range 0-1 */
 
@@ -1656,9 +1648,7 @@ void setup_topology(void) {
 
     nrn_global_ncell = 0;
 
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (Section* sec: range_sec(section_list)) {
 #if 0
 		if (sec->nnode < 1) { /* last node is not a segment */
 			hoc_execerror(secname(sec),
@@ -2200,15 +2190,11 @@ void hoc_level_pushsec(Section* sec) {
 }
 
 void push_section(void) {
-    Section* sec;
+    Section* sec = nullptr;
     if (hoc_is_str_arg(1)) {
-        Item* qsec;
         char* s;
-        sec = (Section*) 0;
         s = gargstr(1);
-        // ForAllSections(sec1) /* I can't imagine a more inefficient way */
-        ITERATE(qsec, section_list) {
-            Section* sec1 = hocSEC(qsec);
+        for (Section* sec1: range_sec(section_list)) {
             if (strcmp(s, nrn_sec2pysecname(sec1)) == 0) {
                 sec = sec1;
                 break;
diff --git a/src/nrnoc/extcelln.cpp b/src/nrnoc/extcelln.cpp
index d807f2bf47..ccfc4250a1 100644
--- a/src/nrnoc/extcelln.cpp
+++ b/src/nrnoc/extcelln.cpp
@@ -225,9 +225,7 @@ void extnode_free_elements(Extnode* nde) {
 }
 
 static void check_if_extracellular_in_use() {
-    hoc_Item* qsec;
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (const Section* sec: range_sec(section_list)) {
         if (sec->pnode[0]->extnode) {
             hoc_execerror("Cannot change nlayer_extracellular when instances exist", NULL);
         }
@@ -486,14 +484,11 @@ void ext_con_coef(void) /* setup a and b */
 {
     int j, k;
     double dx, area;
-    hoc_Item* qsec;
     Node *nd, **pnd;
     Extnode* nde;
 
     /* temporarily store half segment resistances in rhs */
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (Section* sec: range_sec(section_list)) {
         if (sec->pnode[0]->extnode) {
             dx = section_length(sec) / ((double) (sec->nnode - 1));
             for (j = 0; j < sec->nnode - 1; j++) {
@@ -529,9 +524,7 @@ void ext_con_coef(void) /* setup a and b */
     section connects straight to the point*/
     /* for the near future we always have a last node at x=1 with
     no properties */
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (const Section* sec: range_sec(section_list)) {
         if (sec->pnode[0]->extnode) {
             /* node half resistances in general get added to the
             node and to the node's "child node in the same section".
@@ -549,9 +542,7 @@ void ext_con_coef(void) /* setup a and b */
             }
         }
     }
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (Section* sec: range_sec(section_list)) {
         if (sec->pnode[0]->extnode) {
             /* convert to siemens/cm^2 for all nodes except last
             and microsiemens for last.  This means that a*V = mamps/cm2
@@ -578,9 +569,7 @@ void ext_con_coef(void) /* setup a and b */
         }
     }
     /* now the effect of parent on node equation. */
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (const Section* sec: range_sec(section_list)) {
         if (sec->pnode[0]->extnode) {
             for (j = 0; j < sec->nnode; j++) {
                 nd = sec->pnode[j];
diff --git a/src/nrnoc/solve.cpp b/src/nrnoc/solve.cpp
index a3e55c2535..a42fbe960a 100644
--- a/src/nrnoc/solve.cpp
+++ b/src/nrnoc/solve.cpp
@@ -100,17 +100,13 @@ Section** secorder;
 double debugsolve(void) /* returns solution error */
 {
     short inode;
-    int i;
-    Section *sec, *psec, *ch;
     Node *nd, *pnd, **ndP;
     double err, sum;
 
     /* save parts of matrix that will be destroyed */
-    assert(0)
-        /* need to save the rootnodes too */
-        // ForAllSections(sec)
-        ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    assert(0);
+    /* need to save the rootnodes too */
+    for (const Section* sec: range_sec(section_list)) {
         assert(sec->pnode && sec->nnode);
         for (inode = sec->nnode - 1; inode >= 0; inode--) {
             nd = sec->pnode[inode];
@@ -124,9 +120,7 @@ double debugsolve(void) /* returns solution error */
 
     err = 0.;
     /* need to check the rootnodes too */
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (const Section* sec: range_sec(section_list)) {
         for (inode = sec->nnode - 1; inode >= 0; inode--) {
             ndP = sec->pnode + inode;
             nd = sec->pnode[inode];
@@ -141,10 +135,9 @@ double debugsolve(void) /* returns solution error */
             if (inode < sec->nnode - 1) {
                 sum += NODEA(ndP[1]) * NODERHS(ndP[1]);
             }
-            for (ch = nd->child; ch; ch = ch->sibling) {
-                psec = ch;
-                pnd = psec->pnode[0];
-                assert(pnd && psec->nnode);
+            for (const Section* ch = nd->child; ch; ch = ch->sibling) {
+                pnd = ch->pnode[0];
+                assert(pnd && ch->nnode);
                 sum += NODEA(pnd) * NODERHS(pnd);
             }
             sum -= nd->savrhs;
@@ -291,12 +284,9 @@ static void dashes(Section* sec, int offset, int first);
 
 void nrnhoc_topology(void) /* print the topology of the branched cable */
 {
-    hoc_Item* q;
-
     v_setup_vectors();
     Printf("\n");
-    ITERATE(q, section_list) {
-        Section* sec = (Section*) VOIDITM(q);
+    for (Section* sec: range_sec(section_list)) {
         if (sec->parentsec == (Section*) 0) {
             Printf("|");
             dashes(sec, 0, '-');
@@ -434,10 +424,7 @@ void bksub(NrnThread* _nt) {
 }
 
 void nrn_clear_mark(void) {
-    hoc_Item* qsec;
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (Section* sec: range_sec(section_list)) {
         sec->volatile_mark = 0;
     }
 }
@@ -744,15 +731,10 @@ void section_order(void) /* create a section order consistent */
 {
     int order, isec;
     Section* ch;
-    Section* sec;
-    hoc_Item* qsec;
 
     /* count the sections */
     section_count = 0;
-    /*SUPPRESS 765*/
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (Section* sec: range_sec(section_list)) {
         sec->order = -1;
         ++section_count;
     }
@@ -765,9 +747,7 @@ void section_order(void) /* create a section order consistent */
         secorder = (Section**) emalloc(section_count * sizeof(Section*));
     }
     order = 0;
-    // ForAllSections(sec) /* all the roots first */
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (Section* sec: range_sec(section_list)) {
         if (!sec->parentsec) {
             secorder[order] = sec;
             sec->order = order;
@@ -777,10 +757,7 @@ void section_order(void) /* create a section order consistent */
 
     for (isec = 0; isec < section_count; isec++) {
         if (isec >= order) {
-            // Sections form a loop.
-            // ForAllSections(sec)
-            ITERATE(qsec, section_list) {
-                Section* sec = hocSEC(qsec);
+            for (Section* sec: range_sec(section_list)) {
                 Section *psec, *s = sec;
                 for (psec = sec->parentsec; psec; s = psec, psec = psec->parentsec) {
                     if (!psec || s->order >= 0) {
@@ -798,7 +775,7 @@ void section_order(void) /* create a section order consistent */
                 }
             }
         }
-        sec = secorder[isec];
+        Section* sec = secorder[isec];
         for (ch = sec->child; ch; ch = ch->sibling) {
             secorder[order] = ch;
             ch->order = order;
diff --git a/src/nrnoc/treeset.cpp b/src/nrnoc/treeset.cpp
index f27b02fa4f..e7fc5c78c0 100644
--- a/src/nrnoc/treeset.cpp
+++ b/src/nrnoc/treeset.cpp
@@ -814,7 +814,6 @@ void connection_coef(void) /* setup a and b */
 {
     int j;
     double area;
-    hoc_Item* qsec;
     Node* nd;
 #if RA_WARNING
     extern int nrn_ra_set;
@@ -836,9 +835,7 @@ void connection_coef(void) /* setup a and b */
 #endif
     ++recalc_diam_count_;
     nrn_area_ri_nocount_ = 1;
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (Section* sec: range_sec(section_list)) {
         nrn_area_ri(sec);
     }
     nrn_area_ri_nocount_ = 0;
@@ -858,9 +855,7 @@ void connection_coef(void) /* setup a and b */
         std::fill_n(nt.node_a_storage(), nt.end, 0.0);
         std::fill_n(nt.node_b_storage(), nt.end, 0.0);
     }
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (Section* sec: range_sec(section_list)) {
         // Unnecessary because they are unused, but help when looking at fmatrix.
         if (!sec->parentsec) {
             if (auto* const ptr = nrn_classicalNodeA(sec->parentnode)) {
@@ -889,9 +884,7 @@ void connection_coef(void) /* setup a and b */
         }
     }
     /* now the effect of parent on node equation. */
-    // ForAllSections(sec)
-    ITERATE(qsec, section_list) {
-        Section* sec = hocSEC(qsec);
+    for (const Section* sec: range_sec(section_list)) {
         for (j = 0; j < sec->nnode; j++) {
             nd = sec->pnode[j];
             *nrn_classicalNodeB(nd) = -1.e2 * NODERINV(nd) / NODEAREA(nd);
diff --git a/src/nrnpython/nrnpy_hoc.cpp b/src/nrnpython/nrnpy_hoc.cpp
index 1e68124c8c..0ee57d8289 100644
--- a/src/nrnpython/nrnpy_hoc.cpp
+++ b/src/nrnpython/nrnpy_hoc.cpp
@@ -1646,6 +1646,16 @@ static int araychk(Arrayinfo* a, PyHocObject* po, int ix) {
     return 0;
 }
 
+static Py_ssize_t seclist_count(Object* ho) {
+    assert(ho->ctemplate == hoc_sectionlist_template_);
+    hoc_List* sl = (hoc_List*) (ho->u.this_pointer);
+    Py_ssize_t n = 0;
+    for (hoc_Item* q1 = sl->next; q1 != sl; q1 = q1->next) {
+        n++;
+    }
+    return n;
+}
+
 static Py_ssize_t hocobj_len(PyObject* self) {
     PyHocObject* po = (PyHocObject*) self;
     if (po->type_ == PyHoc::HocObject) {
@@ -1654,8 +1664,7 @@ static Py_ssize_t hocobj_len(PyObject* self) {
         } else if (po->ho_->ctemplate == hoc_list_template_) {
             return ivoc_list_count(po->ho_);
         } else if (po->ho_->ctemplate == hoc_sectionlist_template_) {
-            PyErr_SetString(PyExc_TypeError, "hoc.SectionList has no len()");
-            return -1;
+            return seclist_count(po->ho_);
         }
     } else if (po->type_ == PyHoc::HocArray) {
         Arrayinfo* a = hocobj_aray(po->sym_, po->ho_);
@@ -1682,6 +1691,8 @@ static int hocobj_nonzero(PyObject* self) {
             b = vector_capacity((Vect*) po->ho_->u.this_pointer) > 0;
         } else if (po->ho_->ctemplate == hoc_list_template_) {
             b = ivoc_list_count(po->ho_) > 0;
+        } else if (po->ho_->ctemplate == hoc_sectionlist_template_) {
+            b = seclist_count(po->ho_) > 0;
         }
     } else if (po->type_ == PyHoc::HocArray) {
         Arrayinfo* a = hocobj_aray(po->sym_, po->ho_);
@@ -1715,13 +1726,16 @@ static PyObject* hocobj_iter(PyObject* raw_self) {
 
     nb::object self = nb::borrow(raw_self);
     PyHocObject* po = (PyHocObject*) self.ptr();
-    if (po->type_ == PyHoc::HocObject) {
+    if (po->type_ == PyHoc::HocObject || po->type_ == PyHoc::HocSectionListIterator) {
         if (po->ho_->ctemplate == hoc_vec_template_) {
             return PySeqIter_New(self.ptr());
         } else if (po->ho_->ctemplate == hoc_list_template_) {
             return PySeqIter_New(self.ptr());
         } else if (po->ho_->ctemplate == hoc_sectionlist_template_) {
             // need a clone of self so nested loops do not share iteritem_
+            // The HocSectionListIter arm of the outer 'if' became necessary
+            // at Python-3.13.1 upon which the following body is executed
+            // twice. See https://github.com/python/cpython/issues/127682
             auto po2 = nb::steal(nrnpy_ho2po(po->ho_));
             PyHocObject* pho2 = (PyHocObject*) po2.ptr();
             pho2->type_ = PyHoc::HocSectionListIterator;
diff --git a/src/oc/hoclist.h b/src/oc/hoclist.h
index de9666efa0..3a7036dadc 100644
--- a/src/oc/hoclist.h
+++ b/src/oc/hoclist.h
@@ -47,8 +47,33 @@ struct hoc_Item {
 };
 using hoc_List = hoc_Item;
 
-#define ITEM0 (hoc_Item*) 0
-#define LIST0 (hoc_List*) 0
+constexpr auto range_sec(hoc_List* iterable) {
+    struct iterator {
+        hoc_Item* iter;
+        bool operator!=(const iterator& other) const {
+            return iter != other.iter;
+        }
+        void operator++() {
+            iter = iter->next;
+        }
+        Section* operator*() const {
+            return iter->element.sec;
+        }
+    };
+    struct iterable_wrapper {
+        hoc_List* iterable;
+        auto begin() {
+            return iterator{iterable->next};
+        }
+        auto end() {
+            return iterator{iterable};
+        }
+    };
+    return iterable_wrapper{iterable};
+}
+
+#define ITEM0 nullptr
+#define LIST0 nullptr
 
 #define ITERATE(itm, lst) for (itm = (lst)->next; itm != (lst); itm = itm->next)
 /*
diff --git a/test/hoctests/tests/test_seclist.py b/test/hoctests/tests/test_seclist.py
new file mode 100644
index 0000000000..0f3c86696a
--- /dev/null
+++ b/test/hoctests/tests/test_seclist.py
@@ -0,0 +1,18 @@
+from neuron import h
+
+secs = [h.Section() for _ in range(10)]
+
+
+def test():
+    sl = h.SectionList()
+    sl.allroots()
+    assert len(sl) == len(list(sl))
+    b = [(s1, s2) for s1 in sl for s2 in sl]
+    n = len(sl)
+    for i in range(n):
+        for j in range(n):
+            assert b[i * n + j][0] == b[i + j * n][1]
+    return sl
+
+
+sl = test()