add blog/2020-03-10-pypy.md

Co-authored-by: isuruf <[email protected]>

Author: jaimergp

blog/2020-03-10-pypy.md

@@ -0,0 +1,121 @@
+---
+authors:
+  - isuruf
+tags: [infrastructure]
+---
+
+# PyPy builds on conda-forge
+
+conda-forge now supports PyPy3.6 as the python interpreter in a conda
+environment
+
+Supported platforms are,
+
+> -   Linux-x86_64 (glibc 2.12 or newer)
+> -   OSX-x86_64 (OSX 10.9 or newer)
+> -   Linux-aarch64 (glibc 2.17 or newer)
+> -   Linux-ppc64le (glibc 2.17 or newer)
+
+<!--truncate-->
+
+## How to use PyPy
+
+To use the PyPy builds you can do the following,
+
+```bash
+conda config --set channel_priority strict
+conda create -n pypy pypy
+conda activate pypy
+```
+
+If you don't have a conda installation already, you can use
+[miniforge-pypy3](https://github.com/conda-forge/miniforge#miniforge-pypy3)
+which gives you a conda installation powered by pypy itself.
+
+However as of the writing of this post, not many conda packages can be
+installed into this environment, but noarch packages which do not depend
+on the python version nor the interpreter can be installed. For eg,
+mpmath is a noarch package without any dependencies.
+
+```bash
+conda install mpmath    # succeeds
+conda install numpy     # fails as of March 10, 2020
+```
+
+**UPDATE**: numpy and scipy builds are working as of April 10, 2020.
+
+All python C extensions needs to be rebuilt for the PyPy ABI. This is
+currently on the way and can be tracked at the [status
+page](https://conda-forge.org/status).
+
+## python_abi Package
+
+As part of adding support for PyPy and to keep the older python builds
+working, a python_abi package was added. This defines the abi for the
+python package and any non-noarch python packages will have a dependency
+on this package. Older python downstream packages like numpy had their
+metadata patched to add a CPython ABI. You can ask for a specific python
+ABI.
+
+```bash
+conda install "python_abi=*=*_cp27mu"
+```
+
+If you are using python packages from packages other than defaults, you
+will be able to install python extensions built with CPython ABI into
+PyPy builds as their metadata have not been patched. Solution in this
+case is to hotfix the metadata which is available to only high volume
+conda channel or to rebuild those packages with the new python packages
+and mark the older ones as broken.
+
+Using the newer packages will rerender the following requirement to add
+a dependency on python_abi. For eg,
+
+```yaml
+requirements:
+  host:
+   - python 3.6
+  run:
+   - python
+```
+
+is rendered as,
+
+```yaml
+requirements:
+  host:
+   - python 3.6.10 h9d8adfe_1009_cpython
+  run:
+   - python >=3.6,<3.7.0a0
+   - python_abi 3.6 *_cp36m
+```
+
+whereas,
+
+```yaml
+requirements:
+  host:
+   - python 3.6 *_73_pypy
+  run:
+   - python
+```
+
+is rendered as,
+
+```yaml
+requirements:
+  host:
+   - python 3.6.9 0_73_pypy
+  run:
+   - python >=3.6,<3.7.0a0
+   - python_abi 3.6 *_pypy36_pp73
+```
+
+Note that the PyPy ABI tag has `pp73` at the end which
+indicates that the ABI is stable only for PyPy3.6 7.3.x series.
+
+This opens up the possibility of adding debug builds of python and
+building extension packages using the Python Debug ABI.
+
+Discussion on the PyPy builds can be found in the issue
+[conda-forge/conda-forge.github.io#867](https://github.com/conda-forge/conda-forge.github.io/issues/867).