Ask Your Question

preparser state for regexs

asked 2012-08-30 11:58:18 +0200

oldbrad gravatar image

I would have expected the following to work:

import re

preparser(False)                         # preparser off else we have \\ instead of \
rx=re.compile(r".*(\sHun\s).*")          # in the regex

m = rx.match(r"The Attila the Hun Show") # Find "Hun" in "The Attila the Hun Show"
if m: print "found =>",       # 1 needs to be 1 not '1' here!


In fact I get 'IndexError: no such group'. Using int(1) fixes the problem, but why? The preparser is off!

Even more curious the block works, as is, on every other evaluate! I tried moving the preparser(False) statement to before the import, but that makes no difference.

edit retag flag offensive close merge delete

2 Answers

Sort by ยป oldest newest most voted

answered 2012-08-30 12:57:12 +0200

This works for me from the command line. In the notebook, it fails the first time and succeeds the second time. Indeed, having a notebook cell containing


fails the first time I evaluate it and succeeds the second time. This is what is supposed to happen, though. Consider this, from the command-line:

sage: preparser(False); type(2)

The first thing that happens is that the whole line gets preparsed, so "2" gets turned into "Integer(2)". Then the line gets evaluated, so it turns the preparser off and it returns the type of "Integer(2)", namely sage.rings.integer.Integer. I think that the same thing happens in each cell of the notebook: the whole thing gets preparsed before any of it is evaluated. So you need to put preparser(False) in a separate cell and evaluate it first.

edit flag offensive delete link more


No no no .. Something infinitly more diabolical! Experement with two cells as follows Cell one: preparser(False); print type(2) preparser(True); print type(2) Cell two: preparser(True); print type(2) preparser(False); print type(2) preparser(True) Also comment the last and see what happens. I'm totaly confused. I anycase, surely one should leave the preparser in the on state?

oldbrad gravatar imageoldbrad ( 2012-08-30 17:33:16 +0200 )edit

I don't see anything wrong with these two cells. Any "preparser" commands cannot affect the current cell, since they haven't been evaluated yet. Once the cell has been evaluated (in its entirety), they affect any further cells, so for both of your cells, since they both end in "preparser(True)", the preparser is on all the time. If you remove the last "preparser(True)" from cell 2, then after evaluating that cell, the preparser will be off for any subsequent evaluations.

John Palmieri gravatar imageJohn Palmieri ( 2012-08-30 18:48:07 +0200 )edit

You're missing some semicolons, by the way. Cell 1 should be `preparser(False); print type(2); preparser(True); print type(2)`, and cell 2 should be `preparser(True); print type(2); preparser(False); print type(2); preparser(True)`.

John Palmieri gravatar imageJohn Palmieri ( 2012-08-30 18:49:07 +0200 )edit

answered 2018-06-06 12:43:46 +0200

j.c. gravatar image

I had a similar issue dealing with the preparser so I wrote an IPython extension which adds a cell magic %%noparse which disables the Sage preparser for the cell its applied to.

Save this code as

from IPython.core.magic import (Magics, magics_class, cell_magic)

import sage.repl.interpreter

class NoParseMagic(Magics):

    def noparse(self, line, cell):
        Use this cell magic to disable Sage preparsing in an IPython cell.
        sage.repl.interpreter._do_preparse = False
        sage.repl.interpreter._do_preparse = True

def load_ipython_extension(ipython):

If you like, you can copy this to .ipython/extensions/. Then in a Sage Jupyter notebook, you can run:

%load_ext noparse

to load the extension. Now the following works:


which yields:

<type 'int'>

instead of:

<type 'sage.rings.integer.Integer'>
edit flag offensive delete link more


I think this deserves to be added to the sage ipython kernel. Would you mind making a ticket for this?

nbruin gravatar imagenbruin ( 2018-06-06 20:10:48 +0200 )edit

Thanks, I'll see if I can figure out how to do that later tonight.

j.c. gravatar imagej.c. ( 2018-06-06 20:54:26 +0200 )edit
j.c. gravatar imagej.c. ( 2018-06-07 02:20:30 +0200 )edit

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

Question Tools

1 follower


Asked: 2012-08-30 11:58:18 +0200

Seen: 857 times

Last updated: Jun 06 '18