Beginner question: Converting Single-Element tuples to list

[vdavidster]

Hello everyone,

I want to convert a tuple to a list, and I expected this behavior:

list(('abc','def')) -> ['abc','def']
list(('abc')) -> ['abc']

But Python gave me this behavior:

list(('abc','def')) -> ['abc','def']
list(('abc')) -> ['a','b','c']

How do I do get Python to work like the in former case?

Many thanks!

David

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Re: Beginner question: Converting Single-Element tuples to list

[vdavidster]

Hi,

Thanks for your reply! A new thing learned

Allow me to follow that up with another question:

Let's say I have a result from a module called pyparsing:

Results1 = ['abc', 'def']
Results2 = ['abc']

They are of the ParseResults type:

>>> type(Results1)

>>> type(Results2)


I want to convert them into Python lists. list() will work fine on
Results1, but on Results2, it will return:

['a', 'b', 'c']

Because 'abc' is a string. But I want it to return ['abc'].

In short, my question is: how do I typecast an arbitrary object,
whether a list-like object or a string, into a Python list without
having Python split my strings into characters?

Thanks very much for your help!

David

p.s. I know pyparsing has an asList() method, but let's just say I am
unable to use it in my circumstances.

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Re: Beginner question: Converting Single-Element tuples to list

[vdavidster]

Hi Paul and everyone else,

I ran the script and here's what I got:

Python version: 2.4.1 (#2, Mar 31 2005, 00:05:10)
[GCC 3.3 20030304 (Apple Computer, Inc. build 1666)]
pyparsing version: 1.3
'abc def' -> ['abc', 'def']
'abc' -> ['abc']

It seems to work fine.

I figured out what my problem was: my earlier description was
erroneous. The type for Results2 was actually 'str', not
'pyparsing.ParseResults' as I had mentioned. (I was typing from memory
-- I didn't actually run my code sorry, my bad).

It seems that the way I was parsing it, I sometimes got
pyparsing.parseResults lists, and sometimes got  *strings* (not
actually single-element lists as I thought).

I had expected pyparsing results to always return lists, I guess with
my token description below, it doesn't. Here's the bit of code I've
wrote:

from pyparsing import *
unit = Word(alphanums)
section = unit + Literal(':').suppress()
line = ~section + Combine(unit + SkipTo(Literal(';'), include=True))
block = ZeroOrMore(line)
file = Dict(ZeroOrMore(Group(section + block)))
Results = dict(file.parseString(MyFileString)

Where MyFileStrings (the string I want to parse) is something like
this:

var:
x, y, z;
constraints:
x <= 1;
y <= 2;
z <= 3;

And the results I'd get would be something like this:

>>> Results
{'var': 'x, y, z;', 'constraints': (['x <= 1', 'y <= 2', 'z <= 3'],
{})}

So going along Jeff Epler's line of thinking, I wrote a convenience
function:

def convertToList(input):
if type(input) == str:
output = [input]
else:
output = input
return output

So when I iterate through the keys of the Results dictionary, I just
have to convertToList(value) and I will always get a list.

Thanks for your feedback, everyone!

(and embarassingly, I just realized I was talking to the author of
pyparsing. Thanks for pyparsing, Paul! I'm using it to write a
mini-language for defining process control models used in chemical
engineering, and it has really given me a lot of leverage in terms of
churning actual working code out in minimal time. I was thinking of
doing it the lex/yacc way at first, but pyparsing is so much easier.)

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