Python: converting a tuple iterable to a string iterable

Question

I'm getting an iterable of tuples as a result from a sqlite3 select statement, and I want to give this iterable to a function that expects a string iterable. How can I override the next function to give the first index of the tuple? Or to be more precise, what is the right pythonic way of doing this?

>>> res = conn.execute(query,(font,))
>>> train_counts = count_vect.fit_transform(res)

AttributeError: 'tuple' object has no attribute 'lower'

EDIT:

Since mapping involves iterating over the entire list it takes twice as much time as just constructing a generator as Niklas offered.

first = """
l = list()
for i in xrange(10):
    l.append((i,))

for j in (i[0] for i in l):
    j
"""


second = """
l = list()
for i in xrange(10):
    l.append((i,))

convert_to_string = lambda t: "%d" % t
strings = map(convert_to_string, l)

for j in strings:
    j
"""

third = """
l = list()
for i in xrange(10):
    l.append((i,))

strings = [t[0] for t in l]

for j in strings:
    j
"""

print "Niklas B. %f" % timeit.Timer(first).timeit()
print "Richard Fearn %f" % timeit.Timer(second).timeit()
print "Richard Fearn #2 %f" % timeit.Timer(third).timeit()

>>>
Niklas B. 4.744230
Richard Fearn 12.016272
Richard Fearn #2 12.041094

@NiklasB. please post your comment as answer and I'll accept it — zenpoy
– zenpoy, Commented May 12, 2012 at 13:06
Just to be fair: map in Python 3 is just as lazy as a generator expression :) — Niklas B.
– Niklas B., Commented May 12, 2012 at 13:14

Lev Levitsky · Accepted Answer · 2012-05-12 14:40:06Z

4

You need to write a function that will convert each tuple into a string; then you can use map to convert the sequence of tuples into a sequence of strings.

For example:

# assume each tuple contains 3 integers
res = ((1,2,3), (4,5,6))

# converts a 3-integer tuple (x, y, z) to a string with the format "x-y-z"
convert_to_string = lambda t: "%d-%d-%d" % t

# convert each tuple to a string
strings = map(convert_to_string, res)

# call the same function as before, but with a sequence of strings
train_counts = count_vect.fit_transform(strings)

If you want the first item from each tuple, your function could be:

convert_to_string = lambda t: t[0]

(assuming that first element is already a string).

Actually in that case you could avoid the lambda altogether and use a list comprehension:

strings = [t[0] for t in res]

edited May 12, 2012 at 14:40

Lev Levitsky

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answered May 12, 2012 at 12:33

Richard Fearn

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1 Comment

user257111 Over a year ago

You could use a list comprehension for your initial map/lambda bit too: strings = ["%d-%d-%d" % (x,y,z) for x,y,z in res]

Niklas B. · Accepted Answer · 2012-05-12 13:09:42Z

2

The simple solution would be to use a generator expression:

count_vect.fit_transform(t[0] for t in res)

answered May 12, 2012 at 13:09

Niklas B.

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