Anyone reading this will likely already have a background that covers
engineering notation, so I won't go into it deeply. Computer scientists
might also call engineering notation a 'human-readable' numbers. Instead
of 4096B
you might see 4kB
or 4.096kB
, depending on your precision.
There are more expansive explanations all over the web if you want more information.
The Decimal
class is used when you want to do math the way that you
learned to do it in school. Numbers are not represented in the processor
as binary equivalents, they are represented as the exact value. Slower,
but more precise.
The python Decimal
class actually has a built-in method to_eng_string()
appears to be written for this purpose, but actually doesn't work strictly
for all numbers, meaning that:
>>> Decimal('1000000').to_eng_string()
1000000
This is not engineering notation! But it is in line with the standard
that is being implemented by Decimal
. You have to actually give the
string value in scientific notation OR call the normalize
method:
>>> Decimal('1000000').normalize().to_eng_string()
100E+3
Unfortunately, it doesn't work as well for fractional numbers:
>>> Decimal('0.01').to_eng_string()
0.01
>>> Decimal('0.01').normalize().to_eng_string()
0.01
This behavior isn't a fault in the library. As some have explained, the library is doing exactly what it is written to do. It just isn't exactly what I want.
Ideally, I want any number to always resolve into a human-readable form presented in every ENG 101 class:
>>> EngNumber('1000000')
1M
>>> EngNumber('0.1u')
100n
>>> EngNumber('1000m')
1
To get this behavior, consistently, it took writing a bit of code...
As explained above, teh most fundamental features is the ability to resolve any number into engineering notation the same way that you might on paper. Some additional features that would add some usefulness:
int()
and float()
It would be value-added to be able to have units involved as well.
It should be crazy easy to create an EngNumber
:
>>> EngNumber('0.0102')
10.2m
>>> EngNumber('102m')
10.2m
>>> EngNumber('10.2e-2')
10.2m
>>> EngNumber(0.01)
10.2m
>>> EngNumber('10.2k')
10.2k
>>> EngNumber(10200)
10.2k
We should be able to do some easy math with the numbers as well:
>>> EngNumber(10200) + EngNumber('0.4k')
10.6k
>>> EngNumber('10.2e3') - EngNumber(400)
9.8k
Basic comparisons should work out of the box, including comparisons with floats, ints, and Decimals:
>>> EngNumber(10200) == EngNumber('0.4k')
False
>>> EngNumber(10200) >= EngNumber('0.4k')
True
>>> EngNumber(10200) == 10200
True
int()
and float()
Does this need explanation?
>>> n = EngNumber(10200)
>>> int(n)
10200
>>> float(n)
10200.0
Sometimes, adding numbers is just impractical:
>>> EngNumber('10.2k') + EngNumber(1)
10.2k
Source code may be found on github, including 100% test coverage.
Actually, using the Decimal
class as a starting point works very
well since it implements 90% of the desired operations. All we have to
do is parse input strings for proper suffixes (m
, k
, etc.) and
replace __repl__
and __str__
with proper parsing to turn a Decimal
into a human-readable format.
_suffix_lookup = {
'p': 'e-12',
'n': 'e-9',
'u': 'e-6',
'm': 'e-3',
'': 'e0',
'k': 'e3',
'M': 'e6',
'G': 'e9',
'T': 'e12'
}
_exponent_lookup_scaled = {
'-36': 'p',
'-33': 'n',
'-30': 'u',
'-27': 'm',
'-24': '',
'-21': 'k',
'-18': 'M',
'-15': 'G',
'-12': 'T'
}
class EngNumber:
"""
Used for easy manipulation of numbers which use engineering notation
"""
def __init__(self, value: (str, int, float), precision: int=2):
"""
Initialize the class
:param value: string, integer, or float representing the numeric value of the number
:param precision: the precision past the decimal - default to 2
"""
self.precision = precision
if isinstance(value, str):
suffix_keys = [key for key in _suffix_lookup.keys() if key != '']
for suffix in suffix_keys:
if suffix in value:
value = value[:-1] + _suffix_lookup[suffix]
break
self.number = Decimal(value)
elif isinstance(value, int) or isinstance(value, float):
self.number = Decimal(str(value))
def __repr__(self):
"""
Returns the string representation
:return: a string representing the engineering number
"""
# since Decimal class only really converts number that are very small
# into engineering notation, then we will simply make all number a
# small number and take advantage of Decimal class
num_str = self.number * Decimal('10e-25')
num_str = num_str.to_eng_string().lower()
base, exponent = num_str.split('e')
base = str(round(Decimal(base), self.precision))
if '.00' in base:
base = base[:-3]
return base + _exponent_lookup_scaled[exponent]
...
...
One trick that I used in the __repr__
is to take advantage of the
Decimal
class. Since it works well for very small numbers, then
simply multiplying the number by a very small number and looking up
the proper suffix based on the scaled exponent works quite well.
I also created an EngUnit
class which builds on the EngNumber
class.
It simply allows one to specify units that get parsed correctly. It also
does basic unit checking for addition, subtraction, and comparison:
>>> EngUnit('10000ohm')
10kohm
>>> EngUnit('10ohm') + EngUnit(0.02)
AttributeError: units do not match
>>> EngUnit('10ohm') + EngUnit('20ohm')
30ohm
When a unit is not present, then EngUnit
works basically like EngNumber
except the unit is treated as None
.
Enjoy!