How to convert an iterable to a stream?

If I’ve got an iterable containing strings, is there a simple way to turn it into a stream? I want to do something like this:

def make_file():
    yield "hellon"
    yield "worldn"

output = tarfile.TarFile(…)
stream = iterable_to_stream(make_file())
output.addfile(…, stream)

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

Python 3 has a new I/O stream API (library docs), replacing the old file-like object protocol. (The new API is also available in Python 2 in the io module, and it’s backwards-compatible with the file-like object protocol.)

Here’s an implementation for the new API, in Python 2 and 3:

import io

def iterable_to_stream(iterable, buffer_size=io.DEFAULT_BUFFER_SIZE):
    """
    Lets you use an iterable (e.g. a generator) that yields bytestrings as a read-only
    input stream.

    The stream implements Python 3's newer I/O API (available in Python 2's io module).
    For efficiency, the stream is buffered.
    """
    class IterStream(io.RawIOBase):
        def __init__(self):
            self.leftover = None
        def readable(self):
            return True
        def readinto(self, b):
            try:
                l = len(b)  # We're supposed to return at most this much
                chunk = self.leftover or next(iterable)
                output, self.leftover = chunk[:l], chunk[l:]
                b[:len(output)] = output
                return len(output)
            except StopIteration:
                return 0    # indicate EOF
    return io.BufferedReader(IterStream(), buffer_size=buffer_size)

Example usage:

with iterable_to_stream(str(x**2).encode('utf8') for x in range(11)) as s:
    print(s.read())

Method 2

Here’s my streaming iterator an experimental branch of urllib3 supporting streaming chunked request via iterables:

class IterStreamer(object):
    """
    File-like streaming iterator.
    """
    def __init__(self, generator):
        self.generator = generator
        self.iterator = iter(generator)
        self.leftover = ''

    def __len__(self):
        return self.generator.__len__()

    def __iter__(self):
        return self.iterator

    def next(self):
        return self.iterator.next()

    def read(self, size):
        data = self.leftover
        count = len(self.leftover)

        if count < size:
            try:
                while count < size:
                    chunk = self.next()
                    data += chunk
                    count += len(chunk)
            except StopIteration:
                pass

        self.leftover = data[size:]

        return data[:size]

Source with context:
https://github.com/shazow/urllib3/blob/filepost-stream/urllib3/filepost.py#L23

Related unit tests:
https://github.com/shazow/urllib3/blob/filepost-stream/test/test_filepost.py#L9

Alas this code hasn’t made it into the stable branch yet as sizeless chunked requests are poorly supported, but it should be a good foundation for what you’re trying to do. See the source link for examples showing how it can be used.

Method 3

Since it doesn’t look like there is a “standard” way of doing it, I’ve banged together a simple implementation:

class iter_to_stream(object):
    def __init__(self, iterable):
        self.buffered = ""
        self.iter = iter(iterable)

    def read(self, size):
        result = ""
        while size > 0:
            data = self.buffered or next(self.iter, None)
            self.buffered = ""
            if data is None:
                break
            size -= len(data)
            if size < 0:
                data, self.buffered = data[:size], data[size:]
            result += data
        return result

Method 4

A starting point:

class iterable_to_stream:
    def __init__(self, iterable):
        self.iter = iter(iterable)

    def read(self):
        try:
            return self.iter.next()
        except StopIteration:
            return ""

Method 5

TarFile takes anything that provides a file-like interface — so you could either use StringIO (io.StringIO if you are using Python 3.X) to yield what you need to TarFile.addfile() or you could create your own class that provides a file-like interface and yields what you need.

Method 6

A bit modified version of a great Mechanical snail‘s answer. Here, readinto(b) implementation makes multiple calls to the the underlying iterator, in order to gather as much as possible amount of bytes for the size of the given writable bytes-like object b.

class IteratorReader(io.RawIOBase):

    def __init__(self, iterator):
        self.iterator = iterator
        self.leftover = []

    def readinto(self, buffer: bytearray) -> Optional[int]:
        size = len(buffer)
        while len(self.leftover) < size:
            try:
                self.leftover.extend(next(self.iterator))
            except StopIteration:
                break

        if len(self.leftover) == 0:
            return 0

        output, self.leftover = self.leftover[:size], self.leftover[size:]
        buffer[:len(output)] = output
        return len(output)

    def readable(self) -> bool:
        return True

and usage:

def iterator1():
    for i in ('a', 'b', 'c', 'd', 'e', 'f', 'g'):
        res = i * 3
        yield res.encode("utf8")


iterreader = IteratorReader(iterator1())
while True:
    r = iterreader.read(4)
    if not r:
        break
    print(r)

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0