Question

这是我的代码：

from decimal import *
a = eval (input ("Pelase, give me a numbre: \n"))
if type(a) not in (int, float, Decimal):
    print ("It's not possible to make a float from a complex number")
else :
    a=float(a)
    print ("Now your number is", a, "and its type is" , type(a))

如果输入只是文本（例如Hello），则会出错。

我想评估它是否为str，并根据该评估向用户提供建议。

Answer 1

def do_input():
    user_input = input("Input a number: ")
    try:
        num = int(user_input)
    except ValueError:
        try:
            num = float(user_input)
        except ValueError:
            print("You didn't input a number")
            num = None
    return num

for _ in range(3):
    a = do_input()
    print("Now your number is", a, "and its type is" , type(a))

输出：

Input a number: 3
Now your number is 3 and its type is <class 'int'>
Input a number: 2.1
Now your number is 2.1 and its type is <class 'float'>
Input a number: ij
You didn't input a number
Now your number is None and its type is <class 'NoneType'>

Answer 2

在python中，字符串就像是“类str的实例”。为了比较输入中的“内容”是否为字符串，您可以制作类似......

的内容

a = input("Put something...")

if isinstance(a, str):
    print("Error caught, a string was given...")
else:
    print ("Now your number is", a, "and its type is" , type(a))

Answer 3

eval函数并没有真正解析给定的字符串作为数字。它evaluates string as a python expression。所以尝试下面提到的两种方法之一：

单程

from decimal import *

a = input("Please, give me a number : \n")
if type(a) not in (int, float, Decimal):
    print("It's not possible to make a float.")
else:
    a = float(a)
    print("Now your number is", a, "and its type is", type(a))

案例1：

Please, give me a number : 
5
Now your number is 5.0 and its type is <class 'float'>

案例2：

Please, give me a number : 
hello
It's not possible to make a float.

另一种方式

try:
    a = float(input("Please, give me a number : \n"))
    print("Now your number is", a, "and its type is", type(a))
except ValueError:
    print("It's not possible to make a float.")

案例1：

Please, give me a number : 
5
Now your number is 5.0 and its type is <class 'float'>

案例2：

Please, give me a number : 
hello
It's not possible to make a float.

表达式参数被解析并作为Python表达式进行求值（技术上讲，条件列表）使用全局变量和本地变量字典作为全局和本地命名空间。如果是全局字典存在且缺少'内置'，当前全局变量被复制在解析表达式之前进入全局变量。这意味着表达通常可以完全访问标准的内置模块和传播受限制的环境。如果是本地词典省略它默认为全局字典。如果两个字典省略，表达式在环境中执行调用eval（）。返回值是评估的结果表达。语法错误报告为异常。例如：
from math import *

def secret_function():
    return "Secret key is 1234"

def function_creator():

    # expression to be evaluated
    expr = raw_input("Enter the function(in terms of x):")

    # variable used in expression
    x = int(raw_input("Enter the value of x:"))

    # evaluating expression
    y = eval(expr)

    # printing evaluated result
    print("y = {}".format(y))

if __name__ == "__main__":
    function_creator()
输出：
Enter the function(in terms of x):x*(x+1)*(x+2)
Enter the value of x:3
y = 60

Answer 4

而不是使用eval（这是相当危险的 - 用户可以输入任何有效的python代码并且它将运行），你应该使用int，并使用try-catch语句以下内容：

while True:
    try:
            a = int(input ("Pelase, give me a numbre: \n"))
            break
    except ValueError:
            print("Not a number!")

有关更多示例，请参阅此处：https://docs.python.org/3/tutorial/errors.html

Answer 5

eval函数并没有真正解析给定的字符串作为数字。它evaluates string as a python expression。

因此eval('2')给出2的事实只是一个巧合，因为2是正确的python表达式，其计算结果为数字。

所以你不应该使用eval来将字符串解析为数字。相反，只需尝试将其解析（转换）为整数，浮点数和十进制（按此顺序），如果您在任何尝试中都没有收到错误，则表示这是指定类型的正确数量。

@ jose-a发布的答案显示了如何做到这一点。

Answer 6

为什么不简单地将您的逻辑封装在try :: except块中，如下所示：

iNumber = input ("Please, enter a Number: \n")
try :
    # TRY CASTING THE ENTERED DATA TO A FLOAT...
    iNumber = float(iNumber)
    print ("Now your number is {} and its type is {}".format(iNumber, type(iNumber)))
except:
    # IF CASTING FAILS, THEN YOU KNOW IT'S A STRING OR SO... 
    # DO SOMETHING - THROW AN EXCEPTION OR WHATEVER...
    print ("Non Numeric Data is not acceptable...")

的更新：
如果你想处理复数输入（就像你在评论中提到的那样）......你可以将上面的代码包装在if - else块中，如下所示：

import re iNumber = input ("Please, enter a Number: \n") # MATCH A SOMEWHAT COMPLEX NUMBER PATTERN if re.match(r"\d{1,}[ \-\+]*\d{1,}[a-z]", iNumber): print("Not possible to convert a complex number to float: {}".format(iNumber)) else: try : # TRY CASTING THE ENTERED DATA TO A FLOAT... iNumber = float(iNumber) print ("Now your number is {} and its type is {}".format(iNumber, type(iNumber))) except: # IF CASTING FAILS, THEN YOU KNOW IT'S A STRING OR SO... # DO SOMETHING - THROW AN EXCEPTION OR WHATEVER... print ("Non Numeric Data is not acceptable...")

如何在Python中评估字符串？

6 个答案: