检测Python字符串是数字还是字母

Detect whether a Python string is a number or a letter

提问by TriskelionKal

How can I detect either numbers or letters in a string? I am aware you use the ASCII codes, but what functions take advantage of them?

如何检测字符串中的数字或字母？我知道您使用 ASCII 代码，但是哪些函数利用了它们？

回答by Moinuddin Quadri

Check if string is positivedigit (integer) and alphabet

检查字符串是否为正数（整数）和字母

You may use str.isdigit()and str.isalpha()to check whether given string is positiveinteger and alphabet respectively.

您可以使用str.isdigit()和str.isalpha()分别检查给定的字符串是否为正整数和字母。

Sample Results:

示例结果：

# For alphabet
>>> 'A'.isdigit()
False
>>> 'A'.isalpha()
True

# For digit
>>> '1'.isdigit()
True
>>> '1'.isalpha()
False

Check for strings as positive/negative - integer/float

检查字符串为正/负 - 整数/浮点数

str.isdigit()returns Falseif the string is a negativenumber or a float number. For example:

str.isdigit()False如果字符串是负数或浮点数，则返回。例如：

# returns `False` for float
>>> '123.3'.isdigit()
False
# returns `False` for negative number
>>> '-123'.isdigit()
False

If you want to also check for the negativeintegers and float, then you may write a custom function to check for it as:

如果您还想检查负整数 andfloat，那么您可以编写一个自定义函数来检查它：

def is_number(n):
    try:
        float(n)   # Type-casting the string to `float`.
                   # If string is not a valid `float`, 
                   # it'll raise `ValueError` exception
    except ValueError:
        return False
    return True

Sample Run:

示例运行：

>>> is_number('123')    # positive integer number
True

>>> is_number('123.4')  # positive float number
True

>>> is_number('-123')   # negative integer number
True

>>> is_number('-123.4') # negative `float` number
True

>>> is_number('abc')    # `False` for "some random" string
False

Discard "NaN" (not a number) strings while checking for number

检查数字时丢弃“NaN”（不是数字）字符串

The above functions will return Truefor the "NAN" (Not a number) string because for Python it is valid float representing it is not a number. For example:

上述函数将返回True“NAN”（不是数字）字符串，因为对于 Python 来说，它是表示它不是数字的有效浮点数。例如：

>>> is_number('NaN')
True

In order to check whether the number is "NaN", you may use math.isnan()as:

为了检查数字是否为“NaN”，您可以使用math.isnan()：

>>> import math
>>> nan_num = float('nan')

>>> math.isnan(nan_num)
True

Or if you don't want to import additional library to check this, then you may simply check it via comparing it with itself using ==. Python returns Falsewhen nanfloat is compared with itself. For example:

或者，如果您不想导入额外的库来检查这一点，那么您可以简单地通过使用==. False当nanfloat 与自身进行比较时Python 返回。例如：

# `nan_num` variable is taken from above example
>>> nan_num == nan_num
False

Hence, above function is_numbercan be updated to return Falsefor "NaN"as:

因此，可以更新上述函数is_number以返回False为"NaN"：

def is_number(n):
    is_number = True
    try:
        num = float(n)
        # check for "nan" floats
        is_number = num == num   # or use `math.isnan(num)`
    except ValueError:
        is_number = False
    return is_number

Sample Run:

示例运行：

>>> is_number('Nan')   # not a number "Nan" string
False

>>> is_number('nan')   # not a number string "nan" with all lower cased
False

>>> is_number('123')   # positive integer
True

>>> is_number('-123')  # negative integer
True

>>> is_number('-1.12') # negative `float`
True

>>> is_number('abc')   # "some random" string
False

Allow Complex Number like "1+2j" to be treated as valid number

允许像“1+2j”这样的复数被视为有效数字

The above function will still return you Falsefor the complex numbers. If you want your is_numberfunction to treat complex numbersas valid number, then you need to type cast your passed string to complex()instead of float(). Then your is_numberfunction will look like:

上面的函数仍然会返回你False的复数。如果您希望您的is_number函数将复数视为有效数字，那么您需要将传递的字符串类型转换为complex()而不是float(). 然后你的is_number函数看起来像：

def is_number(n):
    is_number = True
    try:
        #      v type-casting the number here as `complex`, instead of `float`
        num = complex(n)
        is_number = num == num
    except ValueError:
        is_number = False
    return is_number

Sample Run:

示例运行：

>>> is_number('1+2j')    # Valid 
True                     #      : complex number 

>>> is_number('1+ 2j')   # Invalid 
False                    #      : string with space in complex number represetantion
                         #        is treated as invalid complex number

>>> is_number('123')     # Valid
True                     #      : positive integer

>>> is_number('-123')    # Valid 
True                     #      : negative integer

>>> is_number('abc')     # Invalid 
False                    #      : some random string, not a valid number

>>> is_number('nan')     # Invalid
False                    #      : not a number "nan" string

PS: Each operation for each check depending on the type of number comes with additional overhead. Choose the version of is_numberfunction which fits your requirement.

PS：根据号码类型，每次检查的每个操作都会带来额外的开销。选择is_number适合您要求的功能版本。

回答by esquarer

For a string of length 1 you can simply perform isdigit()or isalpha()

对于长度为 1 的字符串，您可以简单地执行isdigit()或isalpha()

If your string length is greater than 1, you can make a function something like..

如果您的字符串长度大于 1，您可以创建一个类似..

def isinteger(a):
    try:
        int(a)
        return True
    except ValueError:
        return False