Python內置函數(22)——float

英文文檔：

class float([x])

Return a floating point number constructed from a number or string x.

If the argument is a string, it should contain a decimal number, optionally preceded by a sign, and optionally embedded in whitespace. The optional sign may be '+' or '-'; a '+' sign has no effect on the value produced. The argument may also be a string representing a NaN (not-a-number), or a positive or negative infinity. More precisely, the input must conform to the following grammar after leading and trailing whitespace characters are removed:

sign           ::=  "+" | "-"
infinity       ::=  "Infinity" | "inf"
nan            ::=  "nan"
numeric_value  ::=  floatnumber | infinity | nan
numeric_string ::=  [sign] numeric_value

Here floatnumber is the form of a Python floating-point literal, described in Floating point literals. Case is not significant, so, for example, “inf”, “Inf”, “INFINITY” and “iNfINity” are all acceptable spellings for positive infinity.

Otherwise, if the argument is an integer or a floating point number, a floating point number with the same value (within Python’s floating point precision) is returned. If the argument is outside the range of a Python float, an OverflowError will be raised.

For a general Python object x, float(x) delegates to x.__float__().

If no argument is given, 0.0 is returned.

說明：

　　1. 函數功能將一個數值或者字元轉換成浮點型數值。

>>> float(3)
3.0
>>> float('3')
3.0

　　2. 不提供參數的時候，返回0.0。

>>> float()
0.0

　　3. 字元串必須能正確轉換成浮點型數值的，否則報錯。

>>> float('3.14.15926')
Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    float('3.14.15926')
ValueError: could not convert string to float: '3.14.15926'

　　4. 字元串中允許出現“+”、“-”兩個符號，兩個符號和數字之間不能出現空格，但是符號前面和數字後面允許出現空格。

>>> float('+3.14') #帶正號
3.14
>>> float('-3.14') #帶負號
-3.14
>>> float('  -3.14  ') #正負號前、數字後可以有空格
-3.14
>>> float('- 3.14') #正負號與數字間不可以有空格
Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    float('- 3.14')
ValueError: could not convert string to float: '- 3.14'

　　5. 有幾個特殊的字元串能正確轉換，"Infinity"或者“inf”(不區分大小寫)，能正確轉換，表示無窮大，可以和“+”、“-”一起使用；“nan”也能正確轉換，表示沒有值。

>>> float('Infinity')
inf
>>> float('inf')
inf

>>> float('inFinIty') #不區分大小寫
inf

>>> float('+inFinIty') #正無窮
inf
>>> float('-inFinIty') #負無窮
-inf

>>> float('nan') #沒有值
nan

　　6. 定義的對象如果要被float函數正確轉換成浮點數，需要定義__float__函數。

>>> class X:
    def __init__(self,score):
        self.score = score

>>> x = X(9.7)
>>> float(x) #不能轉換
Traceback (most recent call last):
  File "<pyshell#20>", line 1, in <module>
    float(x)
TypeError: float() argument must be a string or a number, not 'X'


>>> class X: #重新定義類，加入__float__方法
    def __init__(self,score):
        self.score = score
    def __float__(self):
        return self.score

>>> x = X(9.7)
>>> float(x) #可以轉換
9.7