Convert a number using atof

Question

In Python 3.5, I'd like to convert a German number string to a float using locale.atof with the following code:

import locale
from locale import atof
locale.setlocale(locale.LC_ALL, 'de_DE')

number = atof('17.907,08')

However, this raises a ValueError:

ValueError: could not convert string to float: '17.907.08'

Why? Isn't this exactly what atof() was made for?

Is your number `'17.907.08'`, as in the error, or `'17.907,08''` as in your code? — tobias_k, Feb 17 '18 at 16:58
@tobias_k: The latter, it is `17.907,08`, the other format is only the error message. — Jan, Feb 17 '18 at 17:01
To be clear: the claim is that the error message shows the value differently from the source code? That seems hard to believe. — Karl Knechtel, Jan 14 '23 at 04:03

Ronan Boiteau · Accepted Answer · 2018-02-17T16:26:05.040

6

You can't have more than one dot (.) or comma (,) in your number since both of these symbols are used by atof() to separate the decimal part of your number from its integer part.

Since the dots are not needed for Python to correctly represent your number, you should remove them and only keep the comma:

import locale
from locale import atof
locale.setlocale(locale.LC_ALL, 'de_DE')

string_nb = '17.907,08'
string_nb = string_nb.replace('.', '')
number = atof(string)

edited Feb 17 '18 at 16:26

answered Feb 17 '18 at 16:07

Ronan Boiteau

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It works and answers my question - but how to distinguish which part to remove? I have several numbers, English and German ones. – Jan Feb 17 '18 at 16:19
1

@Jan: What is the difference between English and German numbers? Does `17.907,08` in German become `17,907.08` in English? Or `17907.08`? – Ronan Boiteau Feb 17 '18 at 16:29
`$1,000,000` is one million in English while it would be `1.000.000` in German. – Jan Feb 17 '18 at 17:03
2

In Python 3.10 this manipulation is no longer needed. `atof('17.907,08')` works. – Panagiotis Kanavos May 13 '22 at 08:35

score 1 · Answer 2 · answered Feb 17 '18 at 20:06

Just for future reference - this is what I ended up using:

import locale
from locale import atof

def convert(string, thousands_delim = '.', abbr = 'de_DE.UTF-8'):
    ''' Converts a string to float '''

    locale.setlocale(locale.LC_ALL, abbr)
    try:
        number = atof("".join(string.split(thousands_delim)))
    except ValueError:
        number = None

    return number

You call it like

number = convert('17.907,08')
print(number)
# 17907.08

... or for English numbers:

number = convert('1,000,000', abbr = 'en_US')
print(number)
# 1000000.0

You no longer need that, `atof('17.907,08')` just works in Python 3.10. — Panagiotis Kanavos, May 13 '22 at 08:37

score 0 · Answer 3 · answered May 13 '22 at 08:40

In Python 3.10 the works as expected

>>> from locale import atof
>>> import locale
>>> locale.setlocale( locale.LC_ALL, 'de_DE' )
'de_DE'
>>> atof('17.907,08')
17907.08

Using string manipulations to parse decimals is very fragile and should be avoided when possible. Using the wrong replacement could convert decimals to thousands and vice versa

Convert a number using atof

3 Answers3

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