Add support for reading .gypi files.

build/gn_helpers.py

Index: build/gn_helpers.py
diff --git a/build/gn_helpers.py b/build/gn_helpers.py
new file mode 100644
index 0000000000000000000000000000000000000000..cb7531d9dfa119ef5926c239418ffa04c91e14a9
--- /dev/null
+++ b/build/gn_helpers.py
@@ -0,0 +1,73 @@
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import sys
+
+# Helper functions useful when writing scripts that are run from GN's
+# exec_script function.
+
+class Error(Exception):

[Dirk Pranke, 2014/04/03 22:01:28]

"Error" is kind of a generic concept in python used to distinguish runtime
errors in the interpreter from user/script-level exceptions.

So, I wouldn't use the name "Error" here. I'd probably call it "GNException" or
something clever like that :). It is annoying to have to define your own
exceptions this way, but I don't know of a better alternative.

+  pass
+
+
+def ExceptionAppend(e, msg):

[Dirk Pranke, 2014/04/03 22:01:28]

I think you only use this in one place in gypi_to_gn, right?

I tend to think of Exceptions as const objects, and wrap errors in other errors,
so I wouldn't encourage you to have this function at all (see the comment below
as to how I'd wrap one error in another).

+  """Append a message to the given exception's message."""
+  if not e.args:
+    e.args = (msg,)
+  elif len(e.args) == 1:
+    e.args = (str(e.args[0]) + ' ' + msg,)
+  else:
+    e.args = (str(e.args[0]) + ' ' + msg,) + e.args[1:]
+
+
+def PrintString(value):
+  """Writes a python string as a GN string."""
+  if not isinstance(value, str):
+    raise Error("Argument to PrintList is not a Python list.")

[Dirk Pranke, 2014/04/03 22:01:28]

presumably you mean "Python string" here ...

+  if value.find('\n') >= 0:
+    raise Error("Trying to print a string with a newline in it.")
+  sys.stdout.write('"' + value.replace('"', '\\"') + '"')

[Dirk Pranke, 2014/04/03 22:01:28]

I would probably inline these functions into PrintValue.

+
+
+def PrintList(value):
+  """Writes a python list as a GN list."""
+  if not isinstance(value, list):
+    raise Error("Argument to PrintList is not a Python list.")
+  sys.stdout.write('[ ')
+  for i in value:
+    PrintValue(i, False)
+    sys.stdout.write(', ')
+  sys.stdout.write(']')

[Dirk Pranke, 2014/04/03 22:01:28]

assuming you change this to just return a string, the idiomatic way to do this
would be:

return '[%s]' % ', '.join(ToGNString(v) for v in value)

(i.e., one line).

+
+
+def PrintDict(value):
+  """Writes a python dictionary as a GN scope."""
+  for key in value:
+    if not isinstance(key, str):
+      raise Error("Dictionary key is not a string.")
+    sys.stdout.write(key + " = ")
+    PrintValue(value[key], False)  # No recursive dictionaries.
+    sys.stdout.write('\n')

[Dirk Pranke, 2014/04/03 22:01:28]

similarly this can be:

return '\n'.join(("%s = %s" % (k, ToGNString(v))) for k, v in value.items())

+
+
+def PrintValue(value, allow_dicts = True):
+  """Prints the given value to stdout.

[Dirk Pranke, 2014/04/03 22:01:28]

I would probably separate the string conversion out from the printing, i.e.,
just have this function build up a string and return it, and then rename this to
something like ToGNString().

+
+  allow_dicts indicates if this function will allow converting dictionaries
+  to GN scopes. This is only possible at the top level, you can't nest a
+  GN scope in a list, so this should be set to False for recursive calls."""
+  if isinstance(value, str):
+    PrintString(value)
+  elif isinstance(value, list):

[Dirk Pranke, 2014/04/03 22:01:28]

chromium python style prefers early exits where possible. This is even easier if
you change this to ToGNString(), e.g.:

if isinstance(value, str):
  s = <inlined PrintString computation>
  return s
if isinstance(value, list):
  return '[%s]' % ...
...
assert "unsupported type"

+    PrintList(value)
+  elif isinstance(value, dict):
+    if not allow_dicts:
+      raise Error("Attempting to recursively print a dictionary.")
+    PrintDict(value)
+  elif isinstance(value, int):
+    sys.stdout.write(str(value))
+  else:
+    raise Error("Unsupported type when printing to GN.")
+
+