Python - fixed exponent in scientific notation?

问题

Consider the following Python snippet:

for ix in [0.02, 0.2, 2, 20, 200, 2000]:
  iss=str(ix) + "e9"
  isf=float(iss)
  print(iss + "\t=> " + ("%04.03e" % isf ) + " (" + str(isf) + ")")

It generates the following output:

0.02e9  => 2.000e+07 (20000000.0)
0.2e9   => 2.000e+08 (200000000.0)
2e9     => 2.000e+09 (2000000000.0)
20e9    => 2.000e+10 (20000000000.0)
200e9   => 2.000e+11 (2e+11)
2000e9  => 2.000e+12 (2e+12)

 

My question is - is it possible to "go back" somehow? That is:

2.000e+07 => 0.02e9 
2.000e+08 => 0.2e9
2.000e+09 => 2e9    
2.000e+10 => 20e9   
2.000e+11 => 200e9  
2.000e+12 => 2000e9 

... I'd specify I want the exponent to be "e+09"; and then whatever number I throw at this hypothetic function, returns the number value in that exponent? Would it be possible to specify zero padding for both the whole and the decimal part in each case? (ie. 000.0200e9 and 020.0000e9)?

回答1:

Format it yourself (see Format Specification Mini-Language):

for ix in [.02e9,.2e9,2e9,20e9,200e9,2000e9]:
    print('{:.3e} => {:0=8.3f}e9'.format(ix,ix/1e9))

Output

2.000e+07 => 0000.020e9
2.000e+08 => 0000.200e9
2.000e+09 => 0002.000e9
2.000e+10 => 0020.000e9
2.000e+11 => 0200.000e9
2.000e+12 => 2000.000e9

Explanation

{:0=8.3f} means "zero-pad, pad between the sign and the number, total field width 8, 3 places after the decimal, fixed point format".

回答2:

Ah well, got it:

for ix in [0.02, 0.2, 2, 20, 200, 2000]:
  iss=str(ix) + "e9"
  isf=float(iss)
  isf2=isf/float("1e9")
  isf2s = ("%04.03f" % isf2) + "e9"
  print(iss + "\t=> " + ("%04.03e" % isf ) + " (" + str(isf) + ")" + " -> " + isf2s )

... gives:

0.02e9  => 2.000e+07 (20000000.0) -> 0.020e9
0.2e9   => 2.000e+08 (200000000.0) -> 0.200e9
2e9 => 2.000e+09 (2000000000.0) -> 2.000e9
20e9    => 2.000e+10 (20000000000.0) -> 20.000e9
200e9   => 2.000e+11 (2e+11) -> 200.000e9
2000e9  => 2.000e+12 (2e+12) -> 2000.000e9

Sorry for posting,
Cheers!

回答3:

For anyone still coming across this years later...

You can use python's Decimal with quantize

https://docs.python.org/3.6/library/decimal.html#decimal.Decimal.quantize

来源：https://stackoverflow.com/questions/8262302/python-fixed-exponent-in-scientific-notation