[IronPython] IPython is breathing but there's a compile() problem

Mike Krell mbk.lists at gmail.com
Mon May 25 19:00:08 CEST 2009


Now that 2.6B1 has frames support, I've started playing with IronPython
under IPython again.  I've managed to get a command prompt up (some modules
are missing, but the only crucial one is codeop, which I stole from the
standard distribution).

However, there's a problem with entering multiline code snippets
interactively.  With CPython, this looks like:

In [21]: if 1:
  ....:  if 1:
  ....:

(The indentation looks wrong without a fixed-width font, but you get the
idea.)

With IronPython, the second "if 1:" line blows up with a syntax error.  This
boils down to a difference in the way the compile() builtin works as used by
the codeop module.

I've written this up as a bug at codeplex.  Please vote for the bug here:

http://ironpython.codeplex.com/WorkItem/View.aspx?WorkItemId=22692

It would be awesome if we could have a good IronPython + IPython story
before 2.6 is released!

Below are more details about the problem as described in the bug
description.

   Mike

--------------------------------------------------------------------------------

bug description at codeplex follows

compile() behaves differently than in CPython in the presence of incomplete
multiline code snippets.  Fixing this incompatiblity is necessary for
running IronPython under IPython.

Here is a sample program illustrating the problem.  The program is a
modification of the code used in the standard codeop module by IPython to
determine when to provide a continuation prompt for a multiline snippet.

--------------------------------------------------------------------------------

def testcompile(source, flags):

    err = err1 = err2 = None
    code = code1 = code2 = None

    try:
        code = compile(source, "dummy", "single", flags, 1)
    except SyntaxError, err:
        pass

    try:
        code1 = compile(source + "\n", "dummy", "single", flags, 1)
    except SyntaxError, err1:
        pass

    try:
        code2 = compile(source + "\n\n", "dummy", "single", flags, 1)
    except SyntaxError, err2:
        pass

    print "for source = '%s' and flags = %d" % (source, flags),

    if code:
        print "Syntax valid"
    elif not code1 and repr(err1) == repr(err2):
        print "Syntax error!"
        print
        print "err1:", repr(err1)
        print "err2:", repr(err2)
    else:
        print "Continue on next line"
        print
        print "err1:", repr(err1)
        print "err2:", repr(err2)

    print

# 0x200 is PyCF_DONT_IMPLY_DEDENT

testcompile("if 1:", 0x200)
testcompile("if 1:", 0)

testcompile("if 1:\n  if 1:", 0x200)
testcompile("if 1:\n  if 1:", 0)

--------------------------------------------------------------------------------

Under CPython (2.6.1) the output is:

--------------------------------------------------------------------------------

for source = 'if 1:' and flags = 512 Continue on next line

err1: SyntaxError('unexpected EOF while parsing', ('dummy', 1, 6, 'if
1:\n'))
err2: IndentationError('expected an indented block', ('dummy', 2, 1, '\n'))

for source = 'if 1:' and flags = 0 Continue on next line

err1: SyntaxError('unexpected EOF while parsing', ('dummy', 1, 6, 'if
1:\n'))
err2: IndentationError('expected an indented block', ('dummy', 2, 1, '\n'))

for source = 'if 1:
  if 1:' and flags = 512 Continue on next line

err1: IndentationError('expected an indented block', ('dummy', 2, 8, '  if
1:\n'))
err2: IndentationError('expected an indented block', ('dummy', 3, 1, '\n'))

for source = 'if 1:
  if 1:' and flags = 0 Continue on next line

err1: IndentationError('expected an indented block', ('dummy', 2, 8, '  if
1:\n'))
err2: IndentationError('expected an indented block', ('dummy', 3, 1, '\n'))

--------------------------------------------------------------------------------


In all cases the code correctly outputs "Continue on next line" since both
snippets are incomplete but otherwise valid python.

For IronPython 2.6 Beta 1 the output is:

--------------------------------------------------------------------------------

for source = 'if 1:' and flags = 512 Continue on next line

err1: IndentationError("unexpected token '<eof>'", ('dummy', 2, 1, ''))
err2: IndentationError("unexpected token '<eof>'", ('dummy', 3, 1, ''))

for source = 'if 1:' and flags = 0 Continue on next line

err1: IndentationError("unexpected token '<eof>'", ('dummy', 2, 1, ''))
err2: IndentationError("unexpected token '<eof>'", ('dummy', 3, 1, ''))

for source = 'if 1:
  if 1:' and flags = 512 Syntax error!

err1: IndentationError("unexpected token '<eof>'", ('dummy', 2, 8, '  if
1:\n'))
err2: IndentationError("unexpected token '<eof>'", ('dummy', 2, 8, '  if
1:\n'))

for source = 'if 1:
  if 1:' and flags = 0 Syntax error!

err1: IndentationError("unexpected token '<eof>'", ('dummy', 2, 8, '  if
1:\n'))
err2: IndentationError("unexpected token '<eof>'", ('dummy', 2, 8, '  if
1:\n'))

--------------------------------------------------------------------------------

The second snippet is misinterpreted as being a syntax error instead of
merely incomplete.

This is very similar to the issue described here:

http://www.voidspace.org.uk/python/weblog/arch_d7_2007_09_22.shtml#e834

But it apparently doesn't have to do with PyCF_DONT_IMPLY_DEDENT per se
because the CPython output is the same whether this flag is passed or not.
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