Why is using 'eval' a bad practice?


I use the following class to easily store data of my songs.

class Song:
    """The class to store the details of each song"""
    attsToStore=('Name', 'Artist', 'Album', 'Genre', 'Location')
    def __init__(self):
        for att in self.attsToStore:
            exec 'self.%s=None'%(att.lower()) in locals()
    def setDetail(self, key, val):
        if key in self.attsToStore:
            exec 'self.%s=val'%(key.lower()) in locals()

I feel that this is just much more extensible than writing out an if/else block. However, I have heard that eval is unsafe. Is it? What is the risk? How can I solve the underlying problem in my class (setting attributes of self dynamically) without incurring that risk?

Asked By: Nikwin



Yes, using eval is a bad practice. Just to name a few reasons:

  1. There is almost always a better way to do it
  2. Very dangerous and insecure
  3. Makes debugging difficult
  4. Slow

In your case you can use setattr instead:

class Song:
    """The class to store the details of each song"""
    attsToStore=('Name', 'Artist', 'Album', 'Genre', 'Location')
    def __init__(self):
        for att in self.attsToStore:
            setattr(self, att.lower(), None)
    def setDetail(self, key, val):
        if key in self.attsToStore:
            setattr(self, key.lower(), val)

There are some cases where you have to use eval or exec. But they are rare. Using eval in your case is a bad practice for sure. I’m emphasizing on bad practice because eval and exec are frequently used in the wrong place.

Replying to the comments:

It looks like some disagree that eval is ‘very dangerous and insecure’ in the OP case. That might be true for this specific case but not in general. The question was general and the reasons I listed are true for the general case as well.

Answered By: Nadia Alramli

In this case, yes. Instead of

exec 'self.Foo=val'

you should use the builtin function setattr:

setattr(self, 'Foo', val)
Answered By: Josh Lee

Using eval is weak, not a clearly bad practice.

  1. It violates the “Fundamental Principle of Software”. Your source is not the sum total of what’s executable. In addition to your source, there are the arguments to eval, which must be clearly understood. For this reason, it’s the tool of last resort.

  2. It’s usually a sign of thoughtless design. There’s rarely a good reason for dynamic source code, built on-the-fly. Almost anything can be done with delegation and other OO design techniques.

  3. It leads to relatively slow on-the-fly compilation of small pieces of code. An overhead which can be avoided by using better design patterns.

As a footnote, in the hands of deranged sociopaths, it may not work out well. However, when confronted with deranged sociopathic users or administrators, it’s best to not give them interpreted Python in the first place. In the hands of the truly evil, Python can a liability; eval doesn’t increase the risk at all.

Answered By: S.Lott

It’s worth noting that for the specific problem in question, there are several alternatives to using eval:

The simplest, as noted, is using setattr:

def __init__(self):
    for name in attsToStore:
        setattr(self, name, None)

A less obvious approach is updating the object’s __dict__ object directly. If all you want to do is initialize the attributes to None, then this is less straightforward than the above. But consider this:

def __init__(self, **kwargs):
    for name in self.attsToStore:
       self.__dict__[name] = kwargs.get(name, None)

This allows you to pass keyword arguments to the constructor, e.g.:

s = Song(name='History', artist='The Verve')

It also allows you to make your use of locals() more explicit, e.g.:

s = Song(**locals())

…and, if you really want to assign None to the attributes whose names are found in locals():

s = Song(**dict([(k, None) for k in locals().keys()]))

Another approach to providing an object with default values for a list of attributes is to define the class’s __getattr__ method:

def __getattr__(self, name):
    if name in self.attsToStore:
        return None
    raise NameError, name

This method gets called when the named attribute isn’t found in the normal way. This approach somewhat less straightforward than simply setting the attributes in the constructor or updating the __dict__, but it has the merit of not actually creating the attribute unless it exists, which can pretty substantially reduce the class’s memory usage.

The point of all this: There are lots of reasons, in general, to avoid eval – the security problem of executing code that you don’t control, the practical problem of code you can’t debug, etc. But an even more important reason is that generally, you don’t need to use it. Python exposes so much of its internal mechanisms to the programmer that you rarely really need to write code that writes code.

Answered By: Robert Rossney

Yes, it is:

Hack using Python:

>>> eval(input())
...........   #dir listing

The below code will list all tasks running on a Windows machine.

>>> eval(input())

In Linux:

>>> eval(input())
"__import__('subprocess').Popen(['ps', 'aux'],stdout=__import__('subprocess').PIPE).communicate()[0]"
Answered By: Hackaholic

Other users pointed out how your code can be changed as to not depend on eval; I’ll offer a legitimate use-case for using eval, one that is found even in CPython: testing.

Here’s one example I found in test_unary.py where a test on whether (+|-|~)b'a' raises a TypeError:

def test_bad_types(self):
    for op in '+', '-', '~':
        self.assertRaises(TypeError, eval, op + "b'a'")
        self.assertRaises(TypeError, eval, op + "'a'")

The usage is clearly not bad practice here; you define the input and merely observe behavior. eval is handy for testing.

Take a look at this search for eval, performed on the CPython git repository; testing with eval is heavily used.

When eval() is used to process user-provided input, you enable the user to Drop-to-REPL providing something like this:


You may get away with it, but normally you don’t want vectors for arbitrary code execution in your applications.

Answered By: moooeeeep

In addition to @Nadia Alramli answer, since I am new to Python and was eager to check how using eval will affect the timings, I tried a small program and below were the observations:

#Difference while using print() with eval() and w/o eval() to print an int = 0.528969s per 100000 evals()

from datetime import datetime
def strOfNos():
    s = []
    for x in range(100000):
    return s

for x in strOfNos():
    print(x) #print(eval(x))

#when using eval(int)
#2018-10-29 12:36:08.206022
#2018-10-29 12:36:10.407911
#diff = 2.201889 s

#when using int only
#2018-10-29 12:37:50.022753
#2018-10-29 12:37:51.090045
#diff = 1.67292
Answered By: Lokeshwar Tailor
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