5

I seem to get different outputs:

from StringIO import *

file = open('1.bmp', 'r')

print file.read(), '\n'
print StringIO(file.read()).getvalue()

Why? Is it because StringIO only supports text strings or something?

Joelmc
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3 Answers3

8

When you call file.read(), it will read the entire file into memory. Then, if you call file.read() again on the same file object, it will already have reached the end of the file, so it will only return an empty string.

Instead, try e.g. reopening the file:

from StringIO import *

file = open('1.bmp', 'r')
print file.read(), '\n'
file.close()

file2 = open('1.bmp', 'r')
print StringIO(file2.read()).getvalue()
file2.close()

You can also use the with statement to make that code cleaner:

from StringIO import *

with open('1.bmp', 'r') as file:
    print file.read(), '\n'

with open('1.bmp', 'r') as file2:
    print StringIO(file2.read()).getvalue()

As an aside, I would recommend opening binary files in binary mode: open('1.bmp', 'rb')

Liquid_Fire
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  • Yes, you're right. That didn't completely solve my real world problem, then I figured out I was writing the data in 'w' mode and getting corrupt files, not 'wb'. Now everything works :) – Joelmc Sep 26 '11 at 17:48
  • I think that file.seek(0), proposed by minhee, is a much better approach. – Gallaecio Jun 03 '16 at 15:16
5

The second file.read() actually returns just an empty string. You should do file.seek(0) to rewind the internal file offset.

minhee
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-1

Shouldn't you be using "rb" to open, instead of just "r", since this mode assumes that you'll be processing only ASCII characters and EOFs?

agf
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SpagnumMoss
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  • On some platforms (and everywhere on Python 3) just `r` means binary mode. Also, please also don't add taglines / signatures to your posts. – agf Sep 28 '11 at 17:16