Python內建函數(22)——float，python內建22float

Python內建函數(22)——float，python內建22float

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 | nannumeric_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  ') #加號或減號前、數字後可以有空格-3.14>>> float('- 3.14') #加號或減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