Can one output something other than 'nan' for not a number values?

[Chris Green]

I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'. This would
then make it much easier to handle outputting values from sensors when
not all sensors are present.

So, for example, my battery monitoring program outputs:-

Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - 12.34 volts -0.01 Amps

If the starter battery sensor has failed, or is disconnected, I see:-

Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - nan volts nan Amps


What I would like is for those 'nan' strings to be just a '-' or
something similar.

Obviously I can write conditional code to check for float('nan')
values but is there a neater way with any sort of formatting string or
other sort of cleverness?

[Stefan Ram]

Post by Chris Green
Obviously I can write conditional code to check for float('nan')
values but is there a neater way with any sort of formatting string or

This solution does not take the various formatting specifiers into
account correctly, but shows a broad outline of a possible approach.

main.py

import string
import math

class MyFormatter( string.Formatter ):
def format_field( self, value, format_spec ):
if isinstance( value, float )and value != value: value = '-'
return super().format_field( value, format_spec )

fmt = MyFormatter()
a = math.nan
b = 2.3

print( fmt.format( '{a}, {b}', a=a, b=b))

sys.stdout

-, 2.3

[Lawrence D'Oliveiro]

... and value != value ...

Really?

[Cameron Simpson]

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'. This would
then make it much easier to handle outputting values from sensors when
not all sensors are present.
So, for example, my battery monitoring program outputs:-
Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - 12.34 volts -0.01 Amps
If the starter battery sensor has failed, or is disconnected, I see:-
Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - nan volts nan Amps
What I would like is for those 'nan' strings to be just a '-' or
something similar.
Obviously I can write conditional code to check for float('nan')
values but is there a neater way with any sort of formatting string or
other sort of cleverness?

The simplest thing is probably just a function writing it how you want
it:

def float_s(f):
if isnan(f):
return "-"
return str(f)

and then use eg:

print(f'value is {float_s(value)}')

or whatever fits your code.

Cheers,
Cameron Simpson <***@cskk.id.au>

[Grant Edwards]

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'.

I tried monkey-patching the __format__ method of float, but it's
immutable, so that didnt' work. Is float.__format__ what's used by
f-strings, the % operator, etc.?

--
Grant

[Piergiorgio Sartor]

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'. This would
then make it much easier to handle outputting values from sensors when
not all sensors are present.
So, for example, my battery monitoring program outputs:-
Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - 12.34 volts -0.01 Amps
If the starter battery sensor has failed, or is disconnected, I see:-
Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - nan volts nan Amps
What I would like is for those 'nan' strings to be just a '-' or
something similar.
Obviously I can write conditional code to check for float('nan')
values but is there a neater way with any sort of formatting string or
other sort of cleverness?

Uhm, I cannot see how to avoid conditional code.

Somewhere, function, class, method, there should be
an "if isnan(x)".

You can hide that, but you cannot avoid, I suspect.

bye,

[Grant Edwards]

Post by Cameron Simpson

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'.
[...]
Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - nan volts nan Amps
What I would like is for those 'nan' strings to be just a '-' or
something similar.

The simplest thing is probably just a function writing it how you want
return "-"
return str(f)
print(f'value is {float_s(value)}')
or whatever fits your code.

Except he's obviously using some sort of formatting to control the
number of columns and decimal places, so 'str(f)' is not going to cut
it. Is the basic floating point number formatting functionality seen
when using f-strings or '%' operator part of the float type or is it
part of the f-string and % operator?

--
Grant

[Grant Edwards]

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'.

It would probably help if you told us how you're "outputting" them now
(the Python feaatures/functions used, not the actual output format).

Are you using f-strings, the % operator, str.format(), or ??

I would be tempted to try monkey-patching the float class to override
the __format__ method. I have no idea what side effects that might
have, or if it's even used by the various formatting mechanisms, so
you might end up scraping bits off the walls...

--
Grant

[Chris Green]

Post by Grant Edwards

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'.

It would probably help if you told us how you're "outputting" them now
(the Python feaatures/functions used, not the actual output format).
Are you using f-strings, the % operator, str.format(), or ??
I would be tempted to try monkey-patching the float class to override
the __format__ method. I have no idea what side effects that might
have, or if it's even used by the various formatting mechanisms, so
you might end up scraping bits off the walls...

It's using f'{...}' at the moment.

[Grant Edwards]

Post by Chris Green
It's using f'{...}' at the moment.

Here's a demonstration of how to hook custom code into the f-string
formatting engine. It's brilliantly depraved.

https://stackoverflow.com/questions/55876683/hook-into-the-builtin-python-f-string-format-machinery
You can, but only if you write evil code that probably should
never end up in production software. So let's get started!

I'm not going to integrate it into your library, but I will show
you how to hook into the behavior of f-strings. This is roughly
how it'll work:

1. Write a function that manipulates the bytecode instructions of
code objects to replace FORMAT_VALUE instructions with calls
to a hook function;

2. Customize the import mechanism to make sure that the bytecode
of every module and package (except standard library modules
and site-packages) is modified with that function.

Final code is here:

https://github.com/mivdnber/formathack

[Grant Edwards]

Post by Cameron Simpson

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'. [...]
Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - 12.34 volts -0.01 Amps

The simplest thing is probably just a function writing it how you
return "-"
return str(f)

Since he's obviously using one of the float formatting mechanisms to
control the number of columsn and decimal places, I doubt str(f) will
meet the need.

I tried monkey-patching the float type's __format__ method, but it's
immutable.

Is float.__format__() what's used by f-strings, the '%' operator, etc.?

--
Grant

[Grant Edwards]

Post by Cameron Simpson

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'. [...]
Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - nan volts nan Amps
What I would like is for those 'nan' strings to be just a '-' or
something similar.

The simplest thing is probably just a function writing it how you want
return "-"
return str(f)

He's obviouisly using a formatting feature to control columns and
decimal places, so I doubt that 'str(f)' is going to meet the need.

I tried monkey-patching the __format__ method of the 'float' type, but
it's immutable -- so that didn't work.

Is float.__format__() what's used by f-strings, the % operator, etc.?

--
Grant

[Grant Edwards]

Post by Cameron Simpson

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'. [...]
Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - 12.34 volts -0.01 Amps

The simplest thing is probably just a function writing it how you
return "-"
return str(f)

Since he's obviously using one of the float formatting mechanisms to
control the number of columsn and decimal places, I doubt str(f) will
meet the need.

I tried monkey-patching the float type's __format__ method, but it's
immutable.

Is float.__format__() what's used by f-strings, the '%' operator, etc.?

--
Grant

[Chris Angelico]

On Mon, 19 Feb 2024 at 06:47, Grant Edwards via Python-list

Post by Grant Edwards
I would be tempted to try monkey-patching the float class to override
the __format__ method. I have no idea what side effects that might
have, or if it's even used by the various formatting mechanisms, so
you might end up scraping bits off the walls...

You can try, but you'd have to do it in C - the float type is
immutable in Python.

ChrisA

[dn]

Post by Grant Edwards

Post by Cameron Simpson

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'.
[...]
Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - nan volts nan Amps
What I would like is for those 'nan' strings to be just a '-' or
something similar.

The simplest thing is probably just a function writing it how you want
return "-"
return str(f)
print(f'value is {float_s(value)}')
or whatever fits your code.

Except he's obviously using some sort of formatting to control the
number of columns and decimal places, so 'str(f)' is not going to cut
it. Is the basic floating point number formatting functionality seen
when using f-strings or '%' operator part of the float type or is it
part of the f-string and % operator?

It's part of the PSL's string library: "Format Specification
Mini-Language"
https://docs.python.org/3/library/string.html#format-specification-mini-language

Has the OP stated if we're talking 'Python' or numpy, pandas, ...?

[Chris Green]

Post by dn

Post by Grant Edwards

Post by Cameron Simpson

Post by Chris Green
I'm looking for a simple way to make NaN values output as something
like '-' or even just a space instead of the string 'nan'.
[...]
Battery Voltages and Currents
Leisure Battery - 12.42 volts -0.52 Amps
Starter Battery - nan volts nan Amps
What I would like is for those 'nan' strings to be just a '-' or
something similar.

The simplest thing is probably just a function writing it how you want
return "-"
return str(f)
print(f'value is {float_s(value)}')
or whatever fits your code.

Except he's obviously using some sort of formatting to control the
number of columns and decimal places, so 'str(f)' is not going to cut
it. Is the basic floating point number formatting functionality seen
when using f-strings or '%' operator part of the float type or is it
part of the f-string and % operator?

It's part of the PSL's string library: "Format Specification
Mini-Language"
https://docs.python.org/3/library/string.html#format-specification-mini-language
Has the OP stated if we're talking 'Python' or numpy, pandas, ...?

Just python, on a Raspberry Pi, so currently Python 3.9.2.