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What does Python's eval() do?

Posted by: admin November 1, 2017 Leave a comment

Questions:

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

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

What does it do? Can someone explain?

Answers:

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

eval example (interactive shell):

>>> x = 1
>>> eval('x + 1')
2
>>> eval('x')
1

Questions:
Answers:

eval() 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 eval(input()) and os imported, a person could type into input() os.system('rm -R *') which would delete all your files in your home directory. (Assuming you have a unix system). Using eval() is a security hole. If you need to convert strings to other formats, try to use things that do that, like int().

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

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

Questions:
Answers:

Eval() evaluates the passed string as a Python expression and returns the result. For example, eval(“1 + 1”) interprets and executes the expression “1 + 1” 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 here.

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Answers:

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

running_params = {"Greeting":"Hello "}

fout = open("params.dat",'w')

fout.write(repr(running_params))

fout.close()

Read it out as a variable and edit it:

fin = open("params.dat",'r')

diction=eval(fin.read())

diction["Greeting"]+="world"

fin.close()

print diction

Output:

{‘Greeting’: ‘Hello world’}

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Answers:

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

Try eval(input()) and type "1+1" – this should print 2. Eval evaluates expressions.

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eval(), as the name suggests, evaluates the passed argument.

input() is now raw_input() in python 3.x versions. So the most commonly found example for the use of eval() is its use to provide the functionality that input() 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.

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

In short: eval() evaluates the arguments passed to it and hence eval(‘1 + 1’) returned 2.

Questions:
Answers:

Lot’s of good answers here, but none describe the use of eval in the context of its globals= and locals= kwargs. These can be used to limit the methods that are available through the eval method. For example if you load up a fresh python interpreter the locals() and globals() will be the same and look something like this:

>>> globals()
{'__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__doc__': None,
 '__spec__': None, '__builtins__': <module 'builtins' (built-in)>,
 '__package__': None, '__name__': '__main__'}

There are certainly methods within the builtins module that can do significant damage to a system. But it is possible to block anything and everything we don’t want available. Let’s take an example. Say we want to construct a list to represent a domain of the available cores on a system. For me I have 8 cores so I would want a list [1, 8].

>>>from os import cpu_count()
>>>eval('[1, cpu_count()'])
[1, 8]

Likewise all of __builtins__ is available.

>>>eval('abs(-1)')
1

Ok. So there we see one method we want exposed and an example of one (of many that can be much more insidious) method that we do not want exposed. So lets block everything.

>>>eval('[1, cpu_count()]', {'__builtins__':None}, {})
TypeError: 'NoneType' object is not subscriptable

We have effectively blocked all of the __builtins__ methods and as such brought a level of protection into our system. At this point we can start to add back in methods that we do want exposed.

>>>from os import cpu_count
>>>exposed_methods = {'cpu_count': cpu_count}
>>>eval('cpu_count()', {'__builtins__':None}, exposed_methods)
8
>>>eval('abs(cpu_count())', {'__builtins__':None}, exposed_methods)
TypeError: 'NoneType' object is not subscriptable

Now we have the cpu_count method available while still blocking everything we do not want. In my opinion this is super powerful and clearly from the scope of the other answers not a common implementation. There are numerous uses for something like this and as long as it is handled correctly I personally feel eval can be safely used to great value.

N.B.

Something else that is cool about these kwargs is that you can start to use shorthand for your code. Let’s say you use eval as part of a pipeline to execute some imported text. The text doesn’t need to have exact code, it can follow some template file format, and still execute anything you’d like. For example:

>>>from os import cpu_count
>>>eval('[1,cores]', {'__builtins__': None}, {'cores': cpu_count()})
[1, 8]

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eval() makes a string, with integers in it, a mathematical equation (anything in a string is seen as text so integers means numbers as text in this case) so if any help is needed there it is.