DS datatype doesn't accept all valid strings

In the recent switch to using DS derived from decimal.Decimal I had several 
files that stopped being processed properly. 

The resulting OverflowError was:
DS value representation must be <= 16 characters by DICOM standard. Initialize 
with a smaller string, or set config.enforce_valid_values to False to override, 
or use Decimal.quantize() and initialize with a Decimal instance

After looking at the DICOM data directly, the value in question IS actually a 
valid value: '-9.81338674e-006'. This value is <= 16 bytes; however, when 
converted to a decimal.Decimal object, the resulting string becomes: 
'-0.00000981338674' which is now 17 bytes, and thus the DS construction fails.

This is due to the fact that Decimal really doesn't keep track of how long the 
string output is and this particular number fails because Decimal.__str__() 
expands anything with fewer than 6 digits to the left of the decimal sign no 
matter how the decimal was initialized.

As a workaround, I check the length of the input to DS if it's a string and if 
it's a valid length, then I set a flag that enables us to create a DS 
regardless of the Decimal.__str__() length. Additionally, I provided an 
overloaded __str__ method for DS that displays the original string if it was 
provided and Decimal.__str__() otherwise.

Additionally, if the user provides the following string:
'-0.000000981338674'   # Which is not valid by itself (length = 18)

Then a DS can be created because the Decimal.__str__() result is:
'-9.81338674E-7'       # Which is now valid

However, invalid strings that can't be represented by Decimal.decimal in less 
than 16 characters are considered invalid.

I'm not sure if this is the best workaround but I have the patch committed to 
my clone if you wish to take a look. I have also added some tests to ensure 
proper functionality.

http://code.google.com/r/suever-pydicom-dev/source/detail?r=f9fde6290ec8408d1a25
e522036fac86055814f3

-Suever
Original issue reported on code.google.com by Suever@gmail.com on 21 May 2012 at 5:58
Kiyokawa / pydicom

DS datatype doesn't accept all valid strings #116
