在 Python 中判断一个值是否为整数

Question

I would like to determine if a numeric value in Python is a whole number. For example, given:

y = x / 3

I want to distinguish between values of x which are evenly divisible by 3 those which are not.

Answer 1

Integers have no decimals. If you meant "check if a number got decimals in Python", you can do:

not float(your_number).is_integer()

Answer 2

if x % 3 == 0:
    print 'x is divisible by 3'

Answer 3

Edit: As Ollie pointed out in the comment below this post, is_integer is part of the standard library and should therefore not be reimplemented as I did below.

This function uses the fact that every other whole number will have at least one number divisible by two with no remainder. Any non-zero fractional representation in either n or n+1 will cause both n%2 and (n+1)%2 to have a remainder. This has the benefit that whole numbers represented as float values will return True. The function works correctly for positive and negative numbers and zero as far as I can determine. As mentioned in the function, it fails for values very close to an integer.

def isInteger(n):
    """Return True if argument is a whole number, False if argument has a fractional part.

    Note that for values very close to an integer, this test breaks. During
    superficial testing the closest value to zero that evaluated correctly
    was 9.88131291682e-324. When dividing this number by 10, Python 2.7.1 evaluated
    the result to zero"""

    if n%2 == 0 or (n+1)%2 == 0:
        return True
    return False

Answer 4

x % 3 == 0 will be True if x / 3 is an integer.

Answer 5

Here's another method:

x = 1/3  # insert your number here
print(x - int(x) == 0)  # True if x is a whole number, False if it has decimals.

This works because int(x) essentially takes the floor of the number (ex. 3.6453 -> 3). If there's something left over once you subtract the floor, it can't have been a whole number.

Answer 6

assuming you mean if a string containing digits also has a decimal point:

Python 2.6.6 (r266:84292, Apr 20 2011, 11:58:30) 
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> number='123.4'
>>> '.' in number
True
>>> number='123'
>>> '.' in number
False
>>>

To test if it's integral you could mod 1:

>>> 1.0/3 % 1
0.33333333333333331
>>> 1/3 % 1
0

Answer 7

In Python 2, dividing an int by an int returns an int (unless python was invoked with the -Qnew option, or a from __future__ import division is at the beginning of the source; in that case / returns a float); a // specifies integer division.

In Python 3, dividing an int by an int returns a float if you use "/", or an int if you use "//".

If you want to know whether an int will divide into another int exactly, use "%" to look for a remainder.

Answer 8

convert 1.0 => 1 & convert 1.x => 1.x

This code if float numbers has decimal part like 1.5 will return 1.5 & if it is 35.00 it return 35:

a = ReadFrom()    
if float(a).is_integer(): # it is an integer number like 23.00 so return 23
       return int(a)
 else: # for numbers with decimal part like : 1.5 return 1.5
       return float(a)

Answer 9

It is best to make your determination before doing the division, assuming that your x variable is an integer.

Trying to do equality tests or comparisons on floating point numbers is dangerous: http://www.lahey.com/float.htm

The answer already provided using modulus before doing the division to see if one integer is divsible by the other integer is safe. After you do a division and are dealing with possibly floating point values, then numbers are no longer exactly integers or not.