Move python history file to XDG_STATE_HOME

Addresses #17
 - Adds `PYTHONSTARTUP` environment variable to `.zshenv` pointing to
   custom `startup.py` file (bootstraps interactive Python)
 - `startup.py` is symlinked to `XDG_CONFIG_HOME/python` and contains
   cherry-picked section of an open PR that adds XDG support to the
   `site` module (used to customize interactive Python)
 - This is a workaround to redirect the hard-coded history file
   `~/.python_history` to `$XDG_STATE_HOME/python/history`
 - Tested for both system python and virtual environment

Migration takes following steps:
 1. Reload the shell to bring `PYTHONSTARTUP` into scope
 2. Symlink `startup.py`: `make links`
 3. Move/rename the history file (or just drop it):
   `mv ~/.python_history $XDG_STATE_HOME/python/history`

.config/python/startup.py

@@ -0,0 +1,98 @@
+import os
+import sys
+
+# Enable XDG support - cherry-picked from an open PR:
+#  - https://github.com/python/cpython/issues/88405
+#  - https://github.com/python/cpython/blob/dc505f9e9a7a1c4863f0193156bdde9bcfdf8c36/Lib/site.py#L434
+def enablerlcompleter():
+    """Enable default readline configuration on interactive prompts, by
+    registering a sys.__interactivehook__.
+    If the readline module can be imported, the hook will set the Tab key
+    as completion key and register history file.
+    This can be overridden in the sitecustomize or usercustomize module,
+    or in a PYTHONSTARTUP file.
+    """
+    def register_readline():
+        import atexit
+        try:
+            import readline
+            import rlcompleter
+        except ImportError:
+            return
+
+        # Reading the initialization (config) file may not be enough to set a
+        # completion key, so we set one first and then read the file.
+        readline_doc = getattr(readline, '__doc__', '')
+        if readline_doc is not None and 'libedit' in readline_doc:
+            readline.parse_and_bind('bind ^I rl_complete')
+        else:
+            readline.parse_and_bind('tab: complete')
+
+        try:
+            readline.read_init_file()
+        except OSError:
+            # An OSError here could have many causes, but the most likely one
+            # is that there's no .inputrc file (or .editrc file in the case of
+            # Mac OS X + libedit) in the expected location.  In that case, we
+            # want to ignore the exception.
+            pass
+
+        if readline.get_current_history_length() == 0:
+            # If no history was loaded, load history file from
+            # platform defined directories.
+            # The guard is necessary to avoid doubling history size at
+            # each interpreter exit when readline was already configured
+            # through a PYTHONSTARTUP hook, see:
+            # http://bugs.python.org/issue5845#msg198636
+
+            history = get_readline_history_path()
+            history = os.path.abspath(history)
+            try:
+                _dir, _ = os.path.split(history)
+                os.makedirs(_dir, exist_ok=True)
+                readline.read_history_file(history)
+            except OSError:
+                pass
+
+            def write_history():
+                try:
+                    readline.write_history_file(history)
+                except OSError:
+                    # bpo-19891, bpo-41193: Home directory does not exist
+                    # or is not writable, or the filesystem is read-only.
+                    pass
+
+            atexit.register(write_history)
+
+    def get_readline_history_path():
+        def joinuser(*args):
+            return os.path.expanduser(os.path.join(*args))
+
+        # If the legacy path "~/.python_history" is readable, always use it.
+        legacy = joinuser('~', '.python_history')
+        if os.access(legacy, os.R_OK):
+            return legacy
+
+        # Otherwise, use platform defined data directory.
+        if os.name == 'nt':
+            base = os.environ.get('APPDATA') or '~'
+            return joinuser(base, 'Python', 'history')
+
+        if sys.platform == 'darwin':
+            return joinuser('~', 'Library', 'Application Support', 'Python', 'history')
+
+        if os.name == 'posix':
+            base = os.environ.get('XDG_STATE_HOME') or joinuser('~', '.local', 'state')
+            return joinuser(base, 'python', 'history')
+
+        # Unknown platform, use the legacy path.
+        return legacy
+
+    sys.__interactivehook__ = register_readline
+
+# Cherry-picked from
+#  - https://github.com/python/cpython/blob/dc505f9e9a7a1c4863f0193156bdde9bcfdf8c36/Lib/site.py#L636
+#  - https://github.com/python/cpython/blob/dc505f9e9a7a1c4863f0193156bdde9bcfdf8c36/Lib/site.py#L644
+if not sys.flags.no_site and not sys.flags.isolated:
+    enablerlcompleter()
+

.config/zsh/.zshenv

@@ -166,6 +166,11 @@ alias sbt="sbt -ivy ${XDG_DATA_HOME}/ivy2 -sbt-dir ${XDG_DATA_HOME}/sbt"
 #    we actually want to prevent all executions from creating `~/.m2`
 alias mvn="mvn -gs ${XDG_CONFIG_HOME}/maven/settings.xml"
 
+# Python
+#  - Sets path to custom startup file for interactive Python:
+#    https://docs.python.org/3.8/using/cmdline.html#envvar-PYTHONSTARTUP
+export PYTHONSTARTUP=${XDG_CONFIG_HOME}/python/startup.py
+
 # IPython
 #  - Should support XDG directories since version 8.x. Despite the fact that
 #    `$HOME` is still the default, `XDG_CONFIG_HOME` should be picked up.

Makefile

@@ -125,6 +125,7 @@ $(FONTS_DIR) \
 	$(XDG_CONFIG_HOME)/nvim/scripts \
 	$(XDG_CONFIG_HOME)/nvim/vim-plug \
 	$(XDG_CONFIG_HOME)/pypoetry \
+	$(XDG_CONFIG_HOME)/python \
 	$(XDG_DATA_HOME)/npm \
 	$(CRAWL_DIR):
 	mkdir -p $@
@@ -214,6 +215,7 @@ links: \
 	$(XDG_CONFIG_HOME)/nvim/vim-plug \
 	$(XDG_CONFIG_HOME)/nvim/scripts \
 	$(XDG_CONFIG_HOME)/pypoetry \
+	$(XDG_CONFIG_HOME)/python \
 	$(ZDOTDIR) \
 	$(ZSH_CUSTOM)
 	@echo "Linking configuration files:"