How to check if a variable is an integer or a stri

问题:

This question already has an answer here:

• How can I check if a string represents an int, without using try/except? 14 answers

I have an application that has a couple of commands. When you type a certain command, you have to type in additional info about something/someone. Now that info has to be strictly an integer or a string, depending on the situation.

However, whatever you type into Python using raw_input() actually is a string, no matter what, so more specifically, how would I shortly and without try...except see if a variable is made of digits or characters?

回答1:

In my opinion you have two options:

• Just try to convert it to an int, but catch the exception:

  try:
      value = int(value)
  except ValueError:
      pass  # it was a string, not an int.
  

  This is the Ask Forgiveness approach.

• Explicitly test if there are only digits in the string:

  value.isdigit()
  

  str.isdigit() returns True only if all characters in the string are digits (0-9).

  The unicode / Python 3 str type equivalent is unicode.isdecimal() / str.isdecimal(); only Unicode decimals can be converted to integers, as not all digits have an actual integer value (U+00B2 SUPERSCRIPT 2 is a digit, but not a decimal, for example).

  This is often called the Ask Permission approach, or Look Before You Leap.

The latter will not detect all valid int() values, as whitespace and + and - are also allowed in int() values. The first form will happily accept ' +10 ' as a number, the latter won't.

If your expect that the user normally will input an integer, use the first form. It is easier (and faster) to ask for forgiveness rather than for permission in that case.

回答2:

if you want to check what it is:

>>>isinstance(1,str)
False
>>>isinstance('stuff',str)
True
>>>isinstance(1,int)
True
>>>isinstance('stuff',int)
False

if you want to get ints from raw_input

>>>x=raw_input('enter thing:')
enter thing: 3
>>>try: x = int(x)
   except: pass

>>>isinstance(x,int)
True

回答3:

The isdigit method of the str type returns True iff the given string is nothing but one or more digits. If it's not, you know the string should be treated as just a string.

回答4:

Don't check. Go ahead and assume that it is the right input, and catch an exception if it isn't.

intresult = None
while intresult is None:
    input = raw_input()
    try: intresult = int(input)
    except ValueError: pass

回答5:

Depending on your definition of shortly, you could use one of the following options:

• try: int(your_input); except ValueError: # ...
• your_input.isdigit()
• use a regex
• use parse which is kind of the opposite of format