Question

I am trying to print a number into engineering format with python, but I cannot seem to get it to work. The syntax SEEMS simple enough, but it just doesn’t work.

>>> import decimal 
>>> x = decimal.Decimal(1000000)
>>> print x
1000000
>>>> print x.to_eng_string() 
1000000

I cannot figure out why this is. The two values are not equal (one is a string, the other is an int). Setting various contexts in decimal doesn’t seem to help either. Any clues or ideas?

Answer 1

To get this to work, you have to normalize the decimal first:

>>> x = decimal.Decimal ('10000000')

>>> x.normalize()
Decimal('1E+7')

>>> x.normalize().to_eng_string()
'10E+6'

---

The reason for this can be discovered by delving in to the source code.

If you examine to_eng_string() in the Python 2.7.3 source tree (Lib/decimal.py from the gzipped source tar ball here), it simply calls __str__ with eng set to true.

You can then see that it decides on how many digits go to the left of the decimal initially with:

leftdigits = self._exp + len(self._int)

The following table shows what the values are for those two things:

                         ._exp       ._int         len   leftdigits
                         -----       ---------     ---   ----------
Decimal (1000000)            0       '1000000'       7            7
Decimal ('1E+6')             6       '1'             1            7

The code that continues after that is:

if self._exp <= 0 and leftdigits > -6:
    # no exponent required
    dotplace = leftdigits
elif not eng:
    # usual scientific notation: 1 digit on left of the point
    dotplace = 1
elif self._int == '0':
    # engineering notation, zero
    dotplace = (leftdigits + 1) % 3 - 1
else:
    # engineering notation, nonzero
    dotplace = (leftdigits - 1) % 3 + 1

and you can see that, unless it already has an exponent in a certain range (self._exp > 0 or leftdigits <= -6), none will be given to it in the string representation.

---

Further investigation shows the reason for this behaviour. Looking at the code itself, you’ll see it’s based on the General Decimal Arithmetic Specification (PDF here).

If you search that document for to-scientific-string (on which to-engineering-string is heavily based), it states in part (paraphrased, and with my bold bits):

The "to-scientific-string" operation converts a number to a string, using scientific notation if an exponent is needed. The operation is not affected by the context.

If the number is a finite number then:

The coefficient is first converted to a string in base ten using the characters 0 through 9 with no leading zeros (except if its value is zero, in which case a single 0 character is used).

Next, the adjusted exponent is calculated; this is the exponent, plus the number of characters in the converted coefficient, less one. That is, exponent+(clength-1), where clength is the length of the coefficient in decimal digits.

If the exponent is less than or equal to zero and the adjusted exponent is greater than or equal to -6, the number will be converted to a character form without using exponential notation. In this case, if the exponent is zero then no decimal point is added. Otherwise (the exponent will be negative), a decimal point will be inserted with the absolute value of the exponent specifying the number of characters to the right of the decimal point. “0” characters are added to the left of the converted coefficient as necessary. If no character precedes the decimal point after this insertion then a conventional “0” character is prefixed.

In other words, it’s doing what it’s doing because that’s what the standard tells it to do.