blocks|key|644206|text|eval函数允许Python程序在其内部运行Python代码。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|644207|eval示例(交互式shell)：|644208|>>>+x+=+1
>>>+eval('x+%2B+1')
2
>>>+eval('x')
1|code-block|syntax|javascript|644209|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

The eval function lets a Python program run Python code within itself.

eval example (interactive shell):

<pre><code>&gt;&gt;&gt; x = 1
&gt;&gt;&gt; eval('x + 1')
2
&gt;&gt;&gt; eval('x')
1
</code></pre>

blocks|key|644562|text|eval()将字符串解释为代码。这么多人警告您不要使用它的原因是，用户可以使用它作为在计算机上运行代码的选项。如果你导入了eval(input())和os，一个人可以在input()+os.system('rm+-R+*')中输入，这会删除你主目录中的所有文件。(假设您有一个unix系统)。使用eval()是一个安全漏洞。如果您需要将字符串转换为其他格式，请尝试使用类似int()这样的工具。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|644563|entityMap^0|0|6|1P|D|23|2|2C|7|2K|K|44|6|56|5|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|G|$]]

<code>eval()</code> interprets a string as code. The reason why so many people have warned you about using this is because a user can use this as an option to run code on the computer. If you have <code>eval(input())</code> and <code>os</code> imported, a person could type into <code>input()</code> <code>os.system('rm -R *')</code> which would delete all your files in your home directory. (Assuming you have a unix system). Using <code>eval()</code> is a security hole. If you need to convert strings to other formats, try to use things that do that, like <code>int()</code>.

blocks|key|644283|text|在Python2.x中，input(...)等同于eval(raw_input(...))，在Python3.x中，raw_input被重命名为input，我怀疑这会导致您的混淆(您可能正在查看Python2.x中的input文档)。此外，eval(input(...))在Python3.x中可以很好地工作，但在Python2中会引发TypeError。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|644284|在本例中，eval用于将从input返回的字符串强制转换为表达式并进行解释。一般来说，这被认为是不好的做法。|644285|entityMap^0|C|A|P|K|1M|9|20|5|30|5|3C|G|4O|9|0|5|4|D|5|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|I|$]]

In Python 2.x <code>input(...)</code> is equivalent to <code>eval(raw_input(...))</code>, in Python 3.x <code>raw_input</code> was renamed <code>input</code>, which I suspect lead to your confusion (you were probably looking at the documentation for <code>input</code> in Python 2.x). Additionally, <code>eval(input(...))</code> would work fine in Python 3.x, but would raise a <code>TypeError</code> in Python 2.

In this case <code>eval</code> is used to coerce the string returned from <code>input</code> into an expression and interpreted. Generally this is considered bad practice.

blocks|key|429787|text|也许这是一个误导性的阅读一行并解释它的例子。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|429788|尝试eval(input())并键入"1%2B1"+-这将打印2。Eval计算表达式。|offset|length|style|CODE|429789|entityMap^0|0|2|D|I|5|T|1|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]|$D|N|E|O|F|G]|$D|P|E|Q|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|R|8|@]|9|@]|A|$]]]|I|$]]

Maybe a misleading example of reading a line and interpreting it.

Try <code>eval(input())</code> and type <code>"1+1"</code> - this should print <code>2</code>. Eval evaluates expressions.

blocks|key|429903|text|eval()将传递的字符串作为Python表达式进行求值并返回结果。例如，eval("1+%2B+1")解释并执行表达式"1+%2B+1"并返回结果(2)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|429904|您可能会感到困惑的一个原因是，您引用的代码包含一定程度的间接性。内部函数调用(input)首先被执行，因此用户会看到"blah“提示。假设它们的响应是"1+%2B+1“(为了清楚起见，添加了引号，在运行程序时不要键入它们)，输入函数返回该字符串，然后将其传递给外部函数(eval)，该函数解释该字符串并返回结果(2)。|429905|阅读更多关于eval+here的信息。|429906|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.python.org/library/functions.html#eval^0|0|6|11|D|1M|7|0|0|B|4|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Y|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|Z|8|@]|D|@$9|10|A|11|1|12]]|E|$]]|$1|J|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|K|$L|$5|M|N|O|E|$P|Q]]]]

<code>eval()</code> evaluates the passed string as a Python expression and returns the result. For example, <code>eval("1 + 1")</code> interprets and executes the expression <code>"1 + 1"</code> and returns the result (2). 

One reason you might be confused is because the code you cited involves a level of indirection. The inner function call (input) gets executed first so the user sees the "blah" prompt. Let's imagine they respond with "1 + 1" (quotes added for clarity, don't type them when running your program), the input function returns that string, which is then passed to the outer function (eval) which interprets the string and returns the result (2).

Read more about eval <a href="http://docs.python.org/library/functions.html#eval" rel="nofollow noreferrer">here</a>.

blocks|key|430029|text|eval()的一个有用的应用是从字符串计算python表达式。例如，从文件中加载字典的字符串表示：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|430030|running_params+=+{"Greeting":"Hello+"}
fout+=+open("params.dat",'w')
fout.write(repr(running_params))
fout.close()|code-block|syntax|javascript|430031|将其作为变量读出并进行编辑：|430032|fin+=+open("params.dat",'r')
diction=eval(fin.read())
diction["Greeting"]%2B="world"
fin.close()
print+diction|430033|输出：|430034|{'Greeting':+'Hello+world'}|430035|entityMap^0|0|6|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|10|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|S|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|T|$]]

One of useful applications of <code>eval()</code> is to evaluate python expressions from string. For example load from file string representation of dictionary:

<pre><code>running_params = {"Greeting":"Hello "}
fout = open("params.dat",'w')
fout.write(repr(running_params))
fout.close()
</code></pre>

Read it out as a variable and edit it:

<pre><code>fin = open("params.dat",'r')
diction=eval(fin.read())
diction["Greeting"]+="world"
fin.close()
print diction
</code></pre>

Output:

<pre><code>{'Greeting': 'Hello world'}
</code></pre>

blocks|key|430155|text|顾名思义，eval()计算传递的参数。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|430156|在Python3.x版本中，raw_input()现在是input()。因此，最常见的eval()用法示例是使用它来提供input()在Python2.x版本中提供的功能。raw_input以字符串的形式返回用户输入的数据，而input计算输入数据的值并返回。|430157|因此，eval(input("bla+bla"))在2.x中复制了input()的功能，即评估用户输入的数据。|430158|简而言之：eval()计算传递给它的参数，因此eval('1+%2B+1')返回2。|430159|entityMap^0|5|6|0|E|B|S|7|17|6|1O|7|0|3|M|X|7|0|5|6|N|D|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|Z|8|@$9|10|A|11|B|C]|$9|12|A|13|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|L|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|M|$]]

<code>eval()</code>, as the name suggests, evaluates the passed argument.

<code>raw_input()</code> is now <code>input()</code> in python 3.x versions. So the most commonly found example for the use of <code>eval()</code> is its use to provide the functionality that <code>input()</code> provided in 2.x version of python. 
raw_input returned the user-entered data as a string, while input evaluated the value of data entered and returned it. 

<code>eval(input("bla bla"))</code> thus replicates the functionality of <code>input()</code> in 2.x, i.e., of evaluating the user-entered data. 

In short: <code>eval()</code> evaluates the arguments passed to it and hence <code>eval('1 + 1')</code> returned 2.

blocks|key|430227|text|我回答这个问题已经晚了，但是，似乎没有人给出明确的答案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|430228|如果用户输入一个数值，input()将返回一个字符串。|offset|length|style|CODE|430229|>>>+input('Enter+a+number:+')
Enter+a+number:+3
>>>+'3'
>>>+input('Enter+a+number:+')
Enter+a+number:+1%2B1
'1%2B1'|code-block|syntax|javascript|430230|因此，eval()将计算返回值(或表达式)，它是一个字符串，并返回整数/浮点数。|430231|>>>+eval(input('Enter+a+number:+'))
Enter+a+number:+1%2B1
2
>>>+
>>>+eval(input('Enter+a+number:+'))
Enter+a+number:+3.14
3.14|430232|当然，这是一种糟糕的做法。在这种情况下，应该使用int()或float()而不是eval()。|430233|>>>+float(input('Enter+a+number:+'))
Enter+a+number:+3.14
3.14|430234|entityMap^0|0|B|7|0|0|3|6|0|0|O|5|U|7|14|6|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|10|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|11|8|@$D|12|E|13|F|G]]|9|@]|A|$]]|$1|O|3|P|5|J|7|14|8|@]|9|@]|A|$K|L]]|$1|Q|3|R|5|6|7|15|8|@$D|16|E|17|F|G]|$D|18|E|19|F|G]|$D|1A|E|1B|F|G]]|9|@]|A|$]]|$1|S|3|T|5|J|7|1C|8|@]|9|@]|A|$K|L]]|$1|U|3|-4|5|6|7|1D|8|@]|9|@]|A|$]]]|V|$]]

I'm late to answer this question but, no one seems to give clear answer to the question.

If an user enters a numeric value, <code>input()</code> will return a string.

<pre><code>&gt;&gt;&gt; input('Enter a number: ')
Enter a number: 3
&gt;&gt;&gt; '3'
&gt;&gt;&gt; input('Enter a number: ')
Enter a number: 1+1
'1+1'
</code></pre>

So, <code>eval()</code> will evaluate returned value (or expression) which is a string and return integer/float.

<pre><code>&gt;&gt;&gt; eval(input('Enter a number: '))
Enter a number: 1+1
2
&gt;&gt;&gt; 
&gt;&gt;&gt; eval(input('Enter a number: '))
Enter a number: 3.14
3.14
</code></pre>

Of cource this is a bad practice. <code>int()</code> or <code>float()</code> should be used instead of <code>eval()</code> in this case.

<pre><code>&gt;&gt;&gt; float(input('Enter a number: '))
Enter a number: 3.14
3.14
</code></pre>

In the book that I am reading on Python, it keeps using the code <code>eval(input('blah'))</code> 

I read the documentation, and I understand it, but I still do not see how it changes the <code>input()</code> function. 

What does it do? Can someone explain?

What does Python's eval() do? 

在我正在读的关于Python的书中，它一直使用代码eval(input('blah'))我阅读了文档，并且我理解它，但是我仍然不明白它如何改变input()函数。是干什么的呢？有人能解释一下吗？

问Python的eval()做了什么？
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