bob gailer wrote:
On 11/28/2011 12:47 PM, James Reynolds wrote:
On Mon, Nov 28, 2011 at 12:32 PM, Mayo Adams <mayoad...@gmail.com
<mailto:mayoad...@gmail.com>> wrote:
I am trying to pass a set of tuple strings from a file to a
function I
have defined. Each tuple is on a separate line, and looks something
like this:
('note',2048)
As already pointed out - this is a string (a representation of a tuple),
not a tuple.
Your code must parse the string to extract the string representations of
the values, then convert as needed to the desired Python values.
Tasks like this are not trivial.
In general, parsing can be a hard problem. In this case though, it is easy to
solve 95% of the problem with a hand-built converter, which may be good enough.
def strto2tuple(s):
"""Convert a string like "('abc', 42)" to a tuple with two items."""
# Ignore leading and trailing whitespace.
s = s.strip()
# Check for round brackets (parentheses), and remove them.
if s[0] != '(' or s[-1] != ')':
raise ValueError('malformed string, missing ( or )')
s = s[1:-1]
# Split the string into exactly two pieces.
# FIXME this assumes that the first item contains no commas.
items = s.split(',')
n = len(items)
if n != 2:
raise ValueError('expected exactly two items but found %d' % n)
a, b = items
# Ignore spaces around each item, e.g. ( 'abc' , 42 ) => ('abc', 42)
a = a.strip()
b = b.strip()
# Make sure that the first item looks like a string.
quotes = '"\'' # FIXME no support for triple quotes yet, or raw strings.
assert len(quotes) == 2
for q in quotes:
if a.startswith(q) and a.endswith(q):
# Don't include the delimiter quotes in the string.
a = a[1:-1]
break
else:
# This executes if we don't hit a break in the for loop.
raise ValueError('mismatched or missing quotes')
assert isinstance(a, str)
# Make sure the second item is an integer.
b = int(b, 0) # Support hex and octal formats too.
return (a, b) # And return a real tuple.
This untested function will convert strings in a file like these:
( 'fe', 1)
( 'fi' ,2 )
("fo",0x03)
( "fum" , 4 )
into proper tuples with a string and a number. Notice that we allow the user
to be sloppy with spaces, but we are strict about quotation marks and brackets.
Our converter function is both a little too strict (e.g. it forbids the user
from including triple-quoted strings) and a little too lax (e.g. it allows
malformed strings like ''abc'). You might not care about these weaknesses. If
you do, you need to move up to a real parser, which is significantly more complex.
--
Steven
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