Skip to content

Instantly share code, notes, and snippets.

@philipbjorge
Last active March 2, 2016 12:10
Show Gist options
  • Save philipbjorge/5166022 to your computer and use it in GitHub Desktop.
Save philipbjorge/5166022 to your computer and use it in GitHub Desktop.
Empty Set Literal Notation for Python - {_}
"""
Python's set data structure is the only one without a literal
notation for an empty set.
http://excess.org/article/2012/11/python-container-literals/
This is an ast transformer to add an empty set notation with the
following form:
empty_set = {_}
Usage:
At the top of your program,
import EmptySetLiteral
# Any module you import after this will have empty set notation.
# It does not work for the current document!!!
"""
import ast
import imp
import sys
import os.path
import types
class EmptySetLiteralTransformer(ast.NodeTransformer):
def visit_Set(self, node):
try:
if len(node.elts) == 1 and \
isinstance(node.elts[0], ast.Name) and \
node.elts[0].id == "_":
node.elts.pop()
except:
# If we ever get an exception, it's because this node
# doesn't match the strict requirements of the literal {_}.
# So we can return the node unmodified.
pass
return node
"""
The following section for hooking into python's import system is
shamelessly stolen from Fredrik Haard's Blaag.
http://blaag.haard.se/Using-the-AST-to-hack-constants-into-Python/
"""
def transform(src):
"""Transforms the given source and return the AST"""
tree = ast.parse(src)
cm = EmptySetLiteralTransformer()
newtree = cm.visit(tree)
return newtree
class Importer(object):
"""Importer to be put on meta_path to instrument any
subsequent imports"""
def __init__(self):
self._cache = {}
def find_module(self, name, path=None):
try:
suffix = name.split('.')[-1]
self._cache[name] = imp.find_module(suffix)
except ImportError:
return None
return self
def load_module(self, name):
"""Load a module and instrument it. Will fallback to default
behaviour if there is no source available, or if the module to
be imported is not in (PY_SOURCE, PY_COMPILED, PY_DIRECTORY)"""
module = types.ModuleType(name) # create empty module object
fd, pathname, (suffix, mode, type_) = self._cache[name]
with fd:
if type_ == imp.PY_SOURCE:
filename = pathname
elif type_ == imp.PY_COMPILED:
filename = pathname[:-1]
elif type_ == imp.PKG_DIRECTORY:
filename = os.path.join(pathname, '__init__.py')
module.__path__ = [pathname]
else:
return imp.load_module(name, fd, pathname,
(suffix, mode, type_))
if not filename == pathname:
try:
with open(filename, 'U') as realfile:
src = realfile.read()
except IOError: # fallback
return imp.load_module(name, fd, pathname,
(suffix, mode, type_))
else:
src = fd.read()
module.__file__ = filename
module = types.ModuleType(name)
inlined = transform(src)
code = compile(inlined, filename, 'exec')
sys.modules[name] = module
exec(code, module.__dict__)
return module
def install_hook():
"""Install the import hook"""
importer = Importer()
sys.meta_path.insert(0, importer)
# Auto install the hook for imports
install_hook()
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment