How do I control number formatting in the python interpreter?

Question

I often use the python interpreter for doing quick numerical calculations and would like all numerical results to be automatically printed using, e.g., exponential notation. Is there a way to set this for the entire session?

For example, I want:

>>> 1.e12
1.0e+12

not:

>>> 1.e12
1000000000000.0

Answer 1

Create a Python script called whatever you want (say mystartup.py) and then set an environment variable PYTHONSTARTUP to the path of this script. Python will then load this script on startup of an interactive session (but not when running scripts). In this script, define a function similar to this:

def _(v):
    if type(v) == type(0.0):
        print "%e" % v
    else:
        print v

Then, in an interactive session:

C:\temp>set PYTHONSTARTUP=mystartup.py

C:\temp>python
ActivePython 2.5.2.2 (ActiveState Software Inc.) based on
Python 2.5.2 (r252:60911, Mar 27 2008, 17:57:18) [MSC v.1310 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> _(1e12)
1.000000e+012
>>> _(14)
14
>>> _(14.0)
1.400000e+001
>>>

Of course, you can define the function to be called whaetver you want and to work exactly however you want.

Even better than this would be to use IPython. It's great, and you can set the number formatting how you want by using result_display.when_type(some_type)(my_print_func) (see the IPython site or search for more details on how to use this).

Answer 2

I believe the right way is to use sys.displayhook. For example, you could add code like this in your PYTHONSTARTUP file:

import builtins
import sys
import numbers


__orig_hook = sys.displayhook

def __displayhook(value):
    if isinstance(value, numbers.Number) and value >= 1e5:
        builtins._ = value
        print("{:e}".format(value))
    else:
        __orig_hook(value)

sys.displayhook = __displayhook

This will display large enough values using the exp syntax. Feel free to modify the threshold as you see fit.

Alternatively you can have the answer printed in both formats for large numbers:

def __displayhook(value):
    __orig_hook(value)
    if isinstance(value, numbers.Number) and value >= 1e5:
        print("{:e}".format(value))

Or you can define yourself another answer variable besides the default _, such as __ (such creativity, I know):

builtins.__ = None
__orig_hook = sys.displayhook

def __displayhook(value):
    if isinstance(value, numbers.Number):
        builtins.__ = "{:e}".format(value)
    __orig_hook(value)

sys.displayhook = __displayhook

... and then display the exp-formatted answer by typing just __.