blocks|key|961102|text|也许换一种方式更好:检查它是否是一个向量。如果是，你做一个点积，在所有其他情况下，你尝试标量乘法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|961103|检查向量很容易，因为它应该是您的向量类类型(或从它继承的)。你也可以先试着做一个点积，如果失败了(=它不是真正的向量)，那么就退回到标量乘法。|961104|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Probably it's better to just do it the other way around: You check if it's a vector. If it is, you do a dot product and in all other cases you attempt scalar multiplication.

Checking for the vector is easy, since it should of your vector class type (or inherited from it). You could also just try first to do a dot-product, and if that fails (= it wasn't really a vector), then fall back to scalar multiplication.

blocks|key|961182|text|只是为了补充一下。也许我们可以使用isinstance和isdigit的组合，如下所示来确定一个值是否是一个数字(int、float等)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|961183|如果isinstance(num1，int)或isinstance(num1，float)或num1.isdigit()：|961184|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Just to add upon.
Perhaps we can use a combination of isinstance and isdigit as follows to find whether a value is a number (int, float, etc)

if isinstance(num1, int) or isinstance(num1 , float) or num1.isdigit():

blocks|key|1175517|text|在实现一个向量类时，我遇到了类似的问题。检查数字的一种方法是只将其转换为1，即使用|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1175518|float(x)|code-block|syntax|javascript|1175519|这应该拒绝x不能转换为数字的情况；但也可以拒绝其他类型的可能有效的类似数字的结构，例如复数。|1175520|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

I had a similar issue, when implementing a sort of vector class. One way to check for a number is to just convert to one, i.e. by using

<pre><code>float(x)
</code></pre>

This should reject cases where x cannot be converted to a number; but may also reject other kinds of number-like structures that could be valid, for example complex numbers.

blocks|key|961301|text|您可以使用isdigit()函数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|961302|>>>+x+=+"01234"
>>>+a.isdigit()
True
>>>+y+=+"1234abcd"
>>>+y.isdigit()
False|code-block|syntax|javascript|961303|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|$]]

You could use the isdigit() function.

<pre><code>&gt;&gt;&gt; x = "01234"
&gt;&gt;&gt; a.isdigit()
True
&gt;&gt;&gt; y = "1234abcd"
&gt;&gt;&gt; y.isdigit()
False
</code></pre>

blocks|key|1175616|text|总结/评估现有方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1175617|Candidate++++%7C+type++++++++++++++++++++++%7C+delnan+%7C+mat+%7C+shrewmouse+%7C+ant6n
-------------------------------------------------------------------------
0++++++++++++%7C+<type+'int'>++++++++++++++%7C++++++1+%7C+++1+%7C++++++++++1+%7C+++++1
0.0++++++++++%7C+<type+'float'>++++++++++++%7C++++++1+%7C+++1+%7C++++++++++1+%7C+++++1
0j+++++++++++%7C+<type+'complex'>++++++++++%7C++++++1+%7C+++1+%7C++++++++++1+%7C+++++0
Decimal('0')+%7C+<class+'decimal.Decimal'>+%7C++++++1+%7C+++0+%7C++++++++++1+%7C+++++1
True+++++++++%7C+<type+'bool'>+++++++++++++%7C++++++1+%7C+++1+%7C++++++++++1+%7C+++++1
False++++++++%7C+<type+'bool'>+++++++++++++%7C++++++1+%7C+++1+%7C++++++++++1+%7C+++++1
''+++++++++++%7C+<type+'str'>++++++++++++++%7C++++++0+%7C+++0+%7C++++++++++0+%7C+++++0
None+++++++++%7C+<type+'NoneType'>+++++++++%7C++++++0+%7C+++0+%7C++++++++++0+%7C+++++0
'0'++++++++++%7C+<type+'str'>++++++++++++++%7C++++++0+%7C+++0+%7C++++++++++0+%7C+++++1
'1'++++++++++%7C+<type+'str'>++++++++++++++%7C++++++0+%7C+++0+%7C++++++++++0+%7C+++++1
[]+++++++++++%7C+<type+'list'>+++++++++++++%7C++++++0+%7C+++0+%7C++++++++++0+%7C+++++0
[1]++++++++++%7C+<type+'list'>+++++++++++++%7C++++++0+%7C+++0+%7C++++++++++0+%7C+++++0
[1,+2]+++++++%7C+<type+'list'>+++++++++++++%7C++++++0+%7C+++0+%7C++++++++++0+%7C+++++0
(1,)+++++++++%7C+<type+'tuple'>++++++++++++%7C++++++0+%7C+++0+%7C++++++++++0+%7C+++++0
(1,+2)+++++++%7C+<type+'tuple'>++++++++++++%7C++++++0+%7C+++0+%7C++++++++++0+%7C+++++0|code-block|syntax|javascript|1175618|(我是通过this+question来到这里的)|offset|length|1175619|代码|1175620|#!/usr/bin/env+python

"""Check+if+a+variable+is+a+number."""

import+decimal


def+delnan_is_number(candidate):
++++import+numbers
++++return+isinstance(candidate,+numbers.Number)


def+mat_is_number(candidate):
++++return+isinstance(candidate,+(int,+long,+float,+complex))


def+shrewmouse_is_number(candidate):
++++try:
++++++++return+0+==+candidate+*+0
++++except:
++++++++return+False


def+ant6n_is_number(candidate):
++++try:
++++++++float(candidate)
++++++++return+True
++++except:
++++++++return+False

#+Test
candidates+=+(0,+0.0,+0j,+decimal.Decimal(0),
++++++++++++++True,+False,+'',+None,+'0',+'1',+[],+[1],+[1,+2],+(1,+),+(1,+2))

methods+=+[delnan_is_number,+mat_is_number,+shrewmouse_is_number,+ant6n_is_number]

print("Candidate++++%7C+type++++++++++++++++++++++%7C+delnan+%7C+mat+%7C+shrewmouse+%7C+ant6n")
print("-------------------------------------------------------------------------")
for+candidate+in+candidates:
++++results+=+[m(candidate)+for+m+in+methods]
++++print("{:<12}+%7C+{:<25}+%7C+{:>6}+%7C+{:>3}+%7C+{:>10}+%7C+{:>5}"
++++++++++.format(repr(candidate),+type(candidate),+*results))|1175621|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/q/4187185/562769^0|0|0|5|D|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@$I|Z|J|10|1|11]]|A|$]]|$1|K|3|L|5|6|7|12|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

To summarize / evaluate existing methods:

<pre><code>Candidate | type | delnan | mat | shrewmouse | ant6n
-------------------------------------------------------------------------
0 | &lt;type 'int'&gt; | 1 | 1 | 1 | 1
0.0 | &lt;type 'float'&gt; | 1 | 1 | 1 | 1
0j | &lt;type 'complex'&gt; | 1 | 1 | 1 | 0
Decimal('0') | &lt;class 'decimal.Decimal'&gt; | 1 | 0 | 1 | 1
True | &lt;type 'bool'&gt; | 1 | 1 | 1 | 1
False | &lt;type 'bool'&gt; | 1 | 1 | 1 | 1
'' | &lt;type 'str'&gt; | 0 | 0 | 0 | 0
None | &lt;type 'NoneType'&gt; | 0 | 0 | 0 | 0
'0' | &lt;type 'str'&gt; | 0 | 0 | 0 | 1
'1' | &lt;type 'str'&gt; | 0 | 0 | 0 | 1
[] | &lt;type 'list'&gt; | 0 | 0 | 0 | 0
[1] | &lt;type 'list'&gt; | 0 | 0 | 0 | 0
[1, 2] | &lt;type 'list'&gt; | 0 | 0 | 0 | 0
(1,) | &lt;type 'tuple'&gt; | 0 | 0 | 0 | 0
(1, 2) | &lt;type 'tuple'&gt; | 0 | 0 | 0 | 0
</code></pre>

(I came here by <a href="https://stackoverflow.com/q/4187185/562769">this question</a>)

<h2>Code</h2>

<pre><code>#!/usr/bin/env python

"""Check if a variable is a number."""

import decimal


def delnan_is_number(candidate):
 import numbers
 return isinstance(candidate, numbers.Number)


def mat_is_number(candidate):
 return isinstance(candidate, (int, long, float, complex))


def shrewmouse_is_number(candidate):
 try:
 return 0 == candidate * 0
 except:
 return False


def ant6n_is_number(candidate):
 try:
 float(candidate)
 return True
 except:
 return False

# Test
candidates = (0, 0.0, 0j, decimal.Decimal(0),
 True, False, '', None, '0', '1', [], [1], [1, 2], (1, ), (1, 2))

methods = [delnan_is_number, mat_is_number, shrewmouse_is_number, ant6n_is_number]

print("Candidate | type | delnan | mat | shrewmouse | ant6n")
print("-------------------------------------------------------------------------")
for candidate in candidates:
 results = [m(candidate) for m in methods]
 print("{:&lt;12} | {:&lt;25} | {:&gt;6} | {:&gt;3} | {:&gt;10} | {:&gt;5}"
 .format(repr(candidate), type(candidate), *results))
</code></pre>

blocks|key|1175644|text|简单明了的方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1175645|obj+=+12345
print(isinstance(obj,int))|code-block|syntax|javascript|1175646|输出：|1175647|True|1175648|如果object是字符串，则返回False：|1175649|obj+=+'some+string'
print(isinstance(obj,int))|1175650|1175651|False|1175652|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|U|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|V|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|W|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|X|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|O|3|H|5|6|7|Z|8|@]|9|@]|A|$]]|$1|P|3|Q|5|D|7|10|8|@]|9|@]|A|$E|F]]|$1|R|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|S|$]]

Short and simple way :

<pre><code>obj = 12345
print(isinstance(obj,int))
</code></pre>

Output :

<pre><code>True
</code></pre>

If the object is a string, 'False' will be returned :

<pre><code>obj = 'some string'
print(isinstance(obj,int))
</code></pre>

Output :

<pre><code>False
</code></pre>

blocks|key|1175688|text|你有一个数据项，比如rec_day，当它被写入文件时将是一个float。但在程序处理期间，它可以是float、int或str类型(+str在初始化新记录时使用，并包含虚拟标志值)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1175689|然后，您可以检查是否有包含以下内容的数字|1175690|++++++++++++++++type(rec_day)+!=+str+|code-block|syntax|javascript|1175691|我以这种方式构建了一个python程序，并将其作为数值检查放入“维护补丁”中。这是Pythonic式的方式吗？很可能不是，因为我曾经用COBOL编程。|1175692|entityMap^0|A|7|U|5|1D|5|1J|3|1N|3|1U|3|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|15|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|P|$]]

You have a data item, say <code>rec_day</code> that when written to a file will be a <code>float</code>. But during program processing it can be either <code>float</code>, <code>int</code> or <code>str</code> type (the <code>str</code> is used when initializing a new record and contains a dummy flag value).

You can then check to see if you have a number with this

<pre><code> type(rec_day) != str 
</code></pre>

I've structured a python program this way and just put in 'maintenance patch' using this as a numeric check. Is it the Pythonic way? Most likely not since I used to program in COBOL.

blocks|key|961467|text|可以在简单的try异常块中实现|type|unstyled|depth|inlineStyleRanges|entityRanges|data|961468|def+check_if_number(str1):
++++try:
++++++++int(float(str1))
++++++++return+'number'
++++except:
++++++++return+'not+a+number'

a+=+check_if_number('32322')
print+(a)
#+number|code-block|syntax|javascript|961469|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|$]]

can be implemented in a simple try exception block
<pre><code>def check_if_number(str1):
 try:
 int(float(str1))
 return 'number'
 except:
 return 'not a number'

a = check_if_number('32322')
print (a)
# number
</code></pre>

Given an arbitrary python object, what's the best way to determine whether it is a number? Here <code>is</code> is defined as <code>acts like a number in certain circumstances</code>.

For example, say you are writing a vector class. If given another vector, you want to find the dot product. If given a scalar, you want to scale the whole vector.

Checking if something is <code>int</code>, <code>float</code>, <code>long</code>, <code>bool</code> is annoying and doesn't cover user-defined objects that might act like numbers. But, checking for <code>__mul__</code>, for example, isn't good enough because the vector class I just described would define <code>__mul__</code>, but it wouldn't be the kind of number I want.

What is the most pythonic way to check if an object is a number?

给定一个任意的python对象，确定它是否是一个数字的最佳方法是什么？这里，is被定义为acts like a number in certain circumstances。例如，假设您正在编写一个向量类。如果给定另一个向量，则需要找到点积。如果给定一个标量，则需要缩放整个向量。检查某些东西是int、float、lo...

问检查一个对象是否是一个数字，最常见的方法是什么？
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