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Using Python2.7 version. Below is my sample code.

import StringIO
import sys

buff = StringIO.StringIO()
buff.write("hello")
print buff.read()

in the above program, read() returns me nothing where as getvalue() returns me "hello". Can anyone help me out in fixing the issue? I need read() because my following code involves reading "n" bytes.

Constantinius
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raj
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2 Answers2

116

You need to reset the buffer position to the beginning. You can do this by doing buff.seek(0).

Every time you read or write to the buffer, the position is advanced by one. Say you start with an empty buffer.

The buffer value is "", the buffer pos is 0. You do buff.write("hello"). Obviously the buffer value is now hello. The buffer position, however, is now 5. When you call read(), there is nothing past position 5 to read! So it returns an empty string.

Joel Cornett
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28
In [38]: out_2 = StringIO.StringIO('not use write') # be initialized to an existing string by passing the string to the constructor

In [39]: out_2.getvalue()
Out[39]: 'not use write'

In [40]: out_2.read()
Out[40]: 'not use write'

or

In [5]: out = StringIO.StringIO()

In [6]: out.write('use write')

In [8]: out.seek(0)

In [9]: out.read()
Out[9]: 'use write'
Vincent Lee
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    out.seek(0) is what was missing for me when I was trying to dump a pickle to StringIO and upload to s3. Once I pushed back to the beginning, my s3 object would be populated properly. – Matthew Nov 17 '17 at 19:00