Creating Stream To Iterate Over From String In Python

I want to create a stream from a string in Python so that it's equivalent to reading the string as if it's read from a text file. something like: for line in open('myfile.txt'): pr

Solution 1:

I want to create a stream from a string in Python so that it's equivalent to reading the string as if it's read from a text file.

Is this the correct/best way to do it?

Yes, unless you really do want it in a list.

If it is intended to be consumed line by line, the way you are doing it makes sense.

StringIO() creates a file-like object.

File objects have a method, .readlines(), which materialize the object as a list. Instead of materializing the data in a list, you can iterate over it, which is more memory light:

# from StringIO import StringIO # Python 2 import
from io import StringIO # Python 3 import

txt = "foo\nbar\nbaz"

Here we append each line into a list, so that we can demonstrate iterating over the file-like object and keeping a handle on the data. (More efficient would be list(file_like_io).

m_1 = []
file_like_io = StringIO(txt)
for line in file_like_io:
    m_1.append(line)

and now:

>>> m_1
['foo\n', 'bar\n', 'baz']

you can return your io to any index point with seek:

>>> file_like_io.seek(0)
>>> file_like_io.tell() # where we are in the object now
0

If you really want it in a list

.readlines() materializes the StringIO iterator as if one did list(io) - this is considered less preferable.

>>> m_2 = file_like_io.readlines() 

And we can see that our results are the same:

>>> m_1 == m_2
True

Keep in mind that it is splitting after the newlines, preserving them in the text as well, so you'll get two newlines for every printed line, double-spacing on print.

Solution 2:

You could roll your own with a simple generator function like this:

def string_stream(s, separators="\n"):
    start = 0
    for end in range(len(s)):
        if s[end] in separators:
            yield s[start:end]
            start = end + 1
    if start < end:
        yield s[start:end+1]

Example usage:

>>> stream = string_stream("foo\tbar\nbaz\n", "\t\n")
>>> for s in stream:
...     print(s)
...
foo
bar
baz

cStringIO may be faster (I haven't tested), but this would give you flexibility in defining/consuming separators.