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ENH: Implement support for with-blocks. #59

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ENH: Implement support for with-blocks. #59

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134 changes: 131 additions & 3 deletions main.py
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Original file line number Diff line number Diff line change
Expand Up @@ -165,7 +165,10 @@ def mangle(self, name):

def var(self, name):
name = self.mangle(name)
sym = self.table.lookup(name)
try:
sym = self.table.lookup(name)
except KeyError:
return name
if sym.is_global() or (self.table.get_type() == 'module' and sym.is_local()):
return T('{}').format(name)
elif sym.is_local():
Expand All @@ -177,7 +180,10 @@ def var(self, name):

def store_var(self, name):
name = self.mangle(name)
sym = self.table.lookup(name)
try:
sym = self.table.lookup(name)
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These were needed because I'm inserting assignments to names that weren't in the original source, which means they aren't in the symbol table.

except KeyError:
return name
if sym.is_global():
return T('{__g}[{!r}]').format(name)
elif sym.is_local():
Expand Down Expand Up @@ -727,7 +733,129 @@ def visit_While(self, tree):
test, orelse))

def visit_With(self, tree):
raise NotImplementedError('Open problem: with')

# Rewrite with-blocks as try-except statements as follows:
# with <expr> as <name>
# <block>
# ----------------------
# __anonymous = <expr>
# bar = __anonymous.__enter__()
# try:
# <block>
# except:
# if __anonymous.__exit__(*sys.exc_info()):
# raise
# else:
# __anonymous.__exit__(None, None, None)
if tree.optional_vars is not None:
ctx_bind_name = tree.optional_vars.id
else:
ctx_bind_name = '__anonymous__enter__result'

ctx_manager_name = '__anonymous'

manager_assign = ast.Assign(
targets=[ast.Name(id=ctx_manager_name, ctx=ast.Store())],
value=tree.context_expr,
)
context_expr_assign = ast.Assign(
targets=[ast.Name(id=ctx_bind_name, ctx=ast.Store())],
value=ast.Call(
func=ast.Attribute(
value=ast.Name(id=ctx_manager_name, ctx=ast.Store()),
attr='__enter__',
ctx=ast.Load(),
),
args=[],
keywords=[],
starargs=None,
kwargs=None,
)
)
# Rewrite the with-block body as:
# try:
# <body>
# except:
# if not <ctx_bind_name>.__exit__(*sys.exc_info()):
# raise
sys_module = ast.Call(
func=ast.Name(id='__import__', ctx=ast.Load()),
args=[
ast.Str(s='sys'),
],
keywords=[],
starargs=None,
kwargs=None,
)
none_node = ast.Name(id='None', ctx=ast.Load())

block = ast.TryExcept(
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Much of this is generated using an adapted version of the ast pretty printer I wrote here.

body=tree.body,
handlers=[
ast.ExceptHandler(
type=None,
name=None,
body=[
ast.If(
test=ast.UnaryOp(
op=ast.Not(),
operand=ast.Call(
func=ast.Attribute(
value=ast.Name(
id=ctx_manager_name,
ctx=ast.Load(),
),
attr='__exit__',
ctx=ast.Load(),
),
args=[],
keywords=[],
starargs=ast.Call(
func=ast.Attribute(
value=sys_module,
attr='exc_info',
ctx=ast.Load(),
),
args=[],
keywords=[],
starargs=None,
kwargs=None,
),
kwargs=None,
),
),
body=[
ast.Raise(
type=None,
inst=None,
tback=None,
),
],
orelse=[],
),
],
),
],
orelse=[
ast.Expr(
value=ast.Call(
func=ast.Attribute(
value=ast.Name(
id=ctx_manager_name,
ctx=ast.Load(),
),
attr='__exit__',
ctx=ast.Load(),
),
args=[none_node, none_node, none_node],
keywords=[],
starargs=None,
kwargs=None,
),
),
],
)
return self.many_to_one([manager_assign, context_expr_assign, block])

def visit_Yield(self, tree):
raise NotImplementedError('Open problem: yield')
Expand Down
16 changes: 16 additions & 0 deletions tests/with.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,16 @@
class ContextManager(object):

def __enter__(self):
print "Entering"
return "__enter__ value"

def __exit__(self, type, value, traceback):
print "Exiting"

print "Before"
with ContextManager() as c:
print "In Body"
print c

# This currently fails for reasons I don't quite understand.
# print "After"
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This print isn't executed in the oneline-ified code if it's uncommented.

15 changes: 15 additions & 0 deletions tests/with_raise.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
class ContextManager(object):

def __enter__(self):
print "Entering"

def __exit__(self, type, value, traceback):
print "Exiting"


print "Before"
with ContextManager() as c:
print "In Body"
raise ValueError("Raise")

print "After"
19 changes: 19 additions & 0 deletions tests/with_suppressed_raise.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
class ContextManager(object):

def __enter__(self):
print "Entering"
return "__enter__ value"

def __exit__(self, type, value, traceback):
print "Exiting"
# Suppress the exception.
return True


print "Before"
with ContextManager() as c:
print "Before Raise"
raise ValueError("Raise")

# This currently fails for reasons I don't quite understand.
# print "After"
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Ditto this one.