blocks|key|1096064|text|EAFP|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1096065|try:
++++x+=+float(a)
except+ValueError:
++++print("You+must+enter+a+number")|code-block|syntax|javascript|1096066|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

EAFP

<pre><code>try:
 x = float(a)
except ValueError:
 print("You must enter a number")
</code></pre>

blocks|key|1096174|text|使用正则表达式。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1096175|import+re

p+=+re.compile('\d%2B(\.\d%2B)?')

a+=+raw_input('How+much+is+1+share+in+that+company?+')

while+p.match(a)+==+None:
++++print+"You+need+to+write+a+number!\n"
++++a+=+raw_input('How+much+is+1+share+in+that+company?+')|code-block|syntax|javascript|1096176|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Use regular expressions.

<pre><code>import re

p = re.compile('\d+(\.\d+)?')

a = raw_input('How much is 1 share in that company? ')

while p.match(a) == None:
 print "You need to write a number!\n"
 a = raw_input('How much is 1 share in that company? ')
</code></pre>

blocks|key|1096365|text|我认为@dan04有正确的方法(EAFP)，但不幸的是，现实世界通常是一个特例，确实需要一些额外的代码来管理事情-所以下面是更详细的，但也更实用(和现实)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1096366|import+sys

while+True:
++++try:
++++++++a+=+raw_input('How+much+is+1+share+in+that+company?+')
++++++++x+=+float(a)
++++++++#+validity+check(s)
++++++++if+x+<+0:+raise+ValueError('share+price+must+be+positive')
++++except+ValueError,+e:
++++++++print("ValueError:+'{}'".format(e))
++++++++print("Please+try+entering+it+again...")
++++except+KeyboardInterrupt:
++++++++sys.exit("\n<terminated+by+user>")
++++except:
++++++++exc_value+=+sys.exc_info()[1]
++++++++exc_class+=+exc_value.__class__.__name__
++++++++print("{}+exception:+'{}'".format(exc_class,+exc_value))
++++++++sys.exit("<fatal+error+encountered>")
++++else:
++++++++break++#+no+exceptions+occurred,+terminate+loop

print("Share+price+entered:+{}".format(x))|code-block|syntax|javascript|1096367|示例用法：|1096368|>+python+numeric_input.py
How+much+is+1+share+in+that+company?+abc
ValueError:+'could+not+convert+string+to+float:+abc'
Please+try+entering+it+again...
How+much+is+1+share+in+that+company?+-1
ValueError:+'share+price+must+be+positive'
Please+try+entering+it+again...
How+much+is+1+share+in+that+company?+9
Share+price+entered:+9.0

>+python+numeric_input.py
How+much+is+1+share+in+that+company?+9.2
Share+price+entered:+9.2|1096369|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I think @dan04 has the right approach (EAFP), but unfortunately the real world is often a special case and some additional code is really required to manage things—so below is a more elaborate, but also a bit more pragmatic (and realistic):

<pre><code>import sys

while True:
 try:
 a = raw_input('How much is 1 share in that company? ')
 x = float(a)
 # validity check(s)
 if x &lt; 0: raise ValueError('share price must be positive')
 except ValueError, e:
 print("ValueError: '{}'".format(e))
 print("Please try entering it again...")
 except KeyboardInterrupt:
 sys.exit("\n&lt;terminated by user&gt;")
 except:
 exc_value = sys.exc_info()[1]
 exc_class = exc_value.__class__.__name__
 print("{} exception: '{}'".format(exc_class, exc_value))
 sys.exit("&lt;fatal error encountered&gt;")
 else:
 break # no exceptions occurred, terminate loop

print("Share price entered: {}".format(x))
</code></pre>

Sample usage:

<pre class="lang-none prettyprint-override"><code>&gt; python numeric_input.py
How much is 1 share in that company? abc
ValueError: 'could not convert string to float: abc'
Please try entering it again...
How much is 1 share in that company? -1
ValueError: 'share price must be positive'
Please try entering it again...
How much is 1 share in that company? 9
Share price entered: 9.0

&gt; python numeric_input.py
How much is 1 share in that company? 9.2
Share price entered: 9.2
</code></pre>

blocks|key|881603|text|现有的答案是正确的，因为更多的Pythonic方法通常是try...except+(即EAFP)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|881604|但是，如果您真的想进行验证，则可以在使用isdigit()之前删除1个小数点。|881605|>>>+"124".replace(".",+"",+1).isdigit()
True
>>>+"12.4".replace(".",+"",+1).isdigit()
True
>>>+"12..4".replace(".",+"",+1).isdigit()
False
>>>+"192.168.1.1".replace(".",+"",+1).isdigit()
False|code-block|syntax|javascript|881606|请注意，这并没有将浮点型与整型区别对待。如果你真的需要的话，你可以添加这个检查。|881607|entityMap^0|S|C|0|K|9|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|T|8|@$9|U|A|V|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|X|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|P|$]]

The existing answers are correct in that the more Pythonic way is usually to <code>try...except</code> (i.e. EAFP).

However, if you really want to do the validation, you could remove exactly 1 decimal point before using <code>isdigit()</code>.

<pre><code>&gt;&gt;&gt; "124".replace(".", "", 1).isdigit()
True
&gt;&gt;&gt; "12.4".replace(".", "", 1).isdigit()
True
&gt;&gt;&gt; "12..4".replace(".", "", 1).isdigit()
False
&gt;&gt;&gt; "192.168.1.1".replace(".", "", 1).isdigit()
False
</code></pre>

Notice that this does not treat floats any different from ints however. You could add that check if you really need it though.

blocks|key|881680|text|s+=+'12.32'
if+s.replace('.',+'').replace('-',+'').isdigit():
++++print(float(s))|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|881681|请注意，这也适用于负floats。|unstyled|offset|length|style|CODE|881682|entityMap^0|0|A|5|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|N|8|@$G|O|H|P|I|J]]|9|@]|A|$]]|$1|K|3|-4|5|F|7|Q|8|@]|9|@]|A|$]]]|L|$]]

<pre><code>s = '12.32'
if s.replace('.', '').replace('-', '').isdigit():
 print(float(s))
</code></pre>

Note that this will work for negative <code>float</code>s as well.

blocks|key|881715|text|基于dan04的答案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|881716|def+isDigit(x):
++++try:
++++++++float(x)
++++++++return+True
++++except+ValueError:
++++++++return+False|code-block|syntax|javascript|881717|用法：|881718|isDigit(3)+++++#+True
isDigit(3.1)+++#+True
isDigit("3")+++#+True
isDigit("3.1")+#+True
isDigit("hi")++#+False|881719|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Building on dan04's answer:

<pre><code>def isDigit(x):
 try:
 float(x)
 return True
 except ValueError:
 return False
</code></pre>

usage:

<pre><code>isDigit(3) # True
isDigit(3.1) # True
isDigit("3") # True
isDigit("3.1") # True
isDigit("hi") # False
</code></pre>

blocks|key|881727|text|import+re

string1+=+"0.5"
string2+=+"0.5a"
string3+=+"a0.5"
string4+=+"a0.5a"

p+=+re.compile(r'\d%2B(\.\d%2B)?$')

if+p.match(string1):
++++print(string1+%2B+"+float+or+int")
else:
++++print(string1+%2B+"+not+float+or+int")

if+p.match(string2):
++++print(string2+%2B+"+float+or+int")
else:
++++print(string2+%2B+"+not+float+or+int")

if+p.match(string3):
++++print(string3+%2B+"+float+or+int")
else:
++++print(string3+%2B+"+not+float+or+int")

if+p.match(string4):
++++print(string4+%2B+"+float+or+int")
else:
++++print(string4+%2B+"+not+float+or+int")

output:
0.5+float+or+int
0.5a+not+float+or+int
a0.5+not+float+or+int
a0.5a+not+float+or+int|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|881728|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>import re

string1 = "0.5"
string2 = "0.5a"
string3 = "a0.5"
string4 = "a0.5a"

p = re.compile(r'\d+(\.\d+)?$')

if p.match(string1):
 print(string1 + " float or int")
else:
 print(string1 + " not float or int")

if p.match(string2):
 print(string2 + " float or int")
else:
 print(string2 + " not float or int")

if p.match(string3):
 print(string3 + " float or int")
else:
 print(string3 + " not float or int")

if p.match(string4):
 print(string4 + " float or int")
else:
 print(string4 + " not float or int")

output:
0.5 float or int
0.5a not float or int
a0.5 not float or int
a0.5a not float or int
</code></pre>

blocks|key|881798|text|如果字符串包含一些特殊字符，如下划线(如'1_1')，则提供的答案失败。下面的函数在我测试的所有情况下都会返回正确的答案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|881799|def+IfStringRepresentsFloat(s):
try:
++++float(s)
++++return+str(float(s))+==+s
except+ValueError:
++++return+False|code-block|syntax|javascript|881800|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

The provided answers fail if the string contains some special characters such as underscore (e.g. '1_1'). The following function returns correct answer in all case that I tested.
<pre><code>def IfStringRepresentsFloat(s):
try:
 float(s)
 return str(float(s)) == s
except ValueError:
 return False
</code></pre>

<pre><code>a = raw_input('How much is 1 share in that company? ')

while not a.isdigit():
 print("You need to write a number!\n")
 a = raw_input('How much is 1 share in that company? ')
</code></pre>

This only works if the user enters an <code>integer</code>, but I want it to work even if they enter a <code>float</code>, but not when they enter a <code>string</code>. 

So the user should be able to enter both <code>9</code> and <code>9.2</code>, but not <code>abc</code>. 

How should I do it?

Using isdigit for floats?

a = raw_input('How much is 1 share in that company? ')while not a.isdigit():    print("You need to write a number!\n")    a = raw_input('How much is 1 share in ...

问是否对浮点数使用isdigit？
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