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Quickly convert strings to number types.

Project Description

Convert strings to numbers quickly.

This module is a Python C extension that will convert strings to numbers much faster than can be done using pure Python. Additionally, if the string cannot be converted, instead of a ValueError the return value can be either the input as-is or a default value.

To achieve this, the module makes some assumptions about the input type (input is int (or long), float, or str (or unicode)), and otherwise a TypeError is raised.

NOTE: The safe_real, safe_float, safe_int, and safe_forceint functions are deprecated as of fastnumbers version >= 0.3.0; fast_real, fast_float, fast_int, and fast_forceint have each been reimplemented to fall back on the “safe” algorithm if overflow or loss of precision is detect and so the separate “safe” functions are no longer needed.


fastnumbers is essentially a fast C implementation of the following Pure Python function:

def fast_float(input, raise_on_invalid=False, default=None):
        return float(input)
    except ValueError:
        if raise_on_invalid:
        return default if default is not None else input

Some example usage:

>>> from fastnumbers import fast_float
>>> # Convert string to a float
>>> fast_float('56.07')
>>> # Unconvertable string returned as-is by default
>>> fast_float('bad input')
'bad input'
>>> # Unconvertable strings can trigger a default value
>>> fast_float('bad input', default=0)
>>> # Integers are converted to floats
>>> fast_float(54)
>>> # The default built-in float behavior can be triggered with
>>> # "raise_on_invalid" set to True.
>>> fast_float('bad input', raise_on_invalid=True) #doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
ValueError: invalid literal for float(): bad input

NOTE: If you need locale-dependent conversions, supply the fastnumbers function of your choice to locale.atof.

import locale
locale.setlocale(locale.LC_ALL, 'de_DE.UTF-8')
print(atof('468,5', func=fast_float))  # Prints 468.5


Just how much faster is fastnumbers than a pure python implementation? Below are the timing results for the *_float functions; please see the Timing Documentation for details into all timing results.

from timeit import timeit
float_try = '''\
def float_try(input):
    """Typical approach to this problem."""
        return float(input)
    except ValueError:
        return input

float_re = '''\
import re
float_match = re.compile(r'[-+]?\d*\.?\d+(?:[eE][-+]?\d+)?$').match
def float_re(input):
    """Alternate approach to this problem."""
        if float_match(input):
            return float(input)
            return input
    except TypeError:
        return float(input)

print('Invalid input:')
print("Try:", timeit('float_try("invalid")', float_try))
print("re:", timeit('float_re("invalid")', float_re))
print("fast", timeit('fast_float("invalid")', 'from fastnumbers import fast_float'))
print('Valid input:')
print("try:", timeit('float_try("56.07")', float_try))
print("re:", timeit('float_re("56.07")', float_re))
print("fast", timeit('fast_float("56.07")', 'from fastnumbers import fast_float'))

The results will be similar to the below, by vary on the system you are on:

Invalid input:
Try: 2.27156710625
re: 0.570491075516
fast 0.173984050751

Valid input:
try: 0.378665924072
re: 1.08740401268
fast 0.204708099365

As you can see, in all cases fastnumbers beats the pure python implementations.

Full Suite of Functions

In addition to fast_float mentioned above, there are also

  • fast_real
  • fast_int
  • fast_forceint
  • isreal
  • isfloat
  • isint
  • isintlike

Please see the API Documentation for full details.


Seth M. Morton


These are the last three entries of the changelog. See the package documentation for the complete changelog.

04-23-2015 v. 0.3.0

  • Updated the fast_* functions to check if an overflow loss of precision has occurred, and if so fall back on the more accurate number conversion method.
  • In response to the above change, the safe_* functions are now deprecated, and internally now use the same code as the fast_* functions.
  • Updated all unit testing to use the hypothesis module, which results in better test coverage.

09-03-2014 v. 0.2.0

  • Added a ‘default’ option to the conversion functions.

08-12-2014 v. 0.1.4

  • Fixed bug where ‘.’ was incorrectly identified as a valid float/int and converted to 0. This bug only applied to the fast_* and is* functions.
  • The method to catch corner-cases like ‘.’, ‘+’, ‘e’, etc. has been reworked to be more general… case-by-case patches should no longer be needed.

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