Using python s eval vs ast literal eval

Question

I have a situation with some code where eval   came up as a possible solution   Now I have never had to use eval   before but  I have come across plenty of information about the potential danger it can cause   That said  I m very wary about using it  My situation is that I have input being given by a user  datamap   input  Provide some data here      Where datamap needs to be a dictionary   I searched around and found that eval   could work this out  I thought that I might be able to check the type of the input before trying to use the data and that would be a viable security precaution  datamap   eval input  Provide some data here     if not isinstance datamap  dict       return  I read through the docs and I am still unclear if this would be safe or not   Does eval evaluate the data as soon as its entered or after the datamap variable is called  Is the ast module s  literal eval   the only safe option

User · Answer

Python s eager in its evaluation  so eval input        Python 3  will evaluate the user s input as soon as it hits the eval  regardless of what you do with the data afterwards  Therefore  this is not safe  especially when you eval user input  Use ast literal eval   As an example  entering this at the prompt could be very bad for you    import    os   system  rm -rf  a-path-you-really-care-about

User · Answer

datamap   eval input  Provide some data here      means that you actually evaluate the code before you deem it to be unsafe or not  It evaluates the code as soon as the function is called  See also the dangers of eval  ast literal eval raises an exception if the input isn t a valid Python datatype  so the code won t be executed if it s not  Use ast literal eval whenever you need eval  You shouldn t usually evaluate literal Python statements

User · Answer

I was stuck with ast literal eval    I was trying it in IntelliJ IDEA debugger  and it kept returning None on debugger output   But later when I assigned its output to a variable and printed it in code  It worked fine  Sharing code example   import ast sample string       id   XYZ GTTC TYR    name   Suction     output value   ast literal eval sample string  print output value    Its python version 3 6

User · Answer

ast literal eval   only considers a small subset of Python s syntax to be valid   The string or node provided may only consist of the following Python literal structures  strings  bytes  numbers  tuples  lists  dicts  sets  booleans  and None   Passing   import    os   system  rm -rf  a-path-you-really-care-about   into ast literal eval   will raise an error  but eval   will happily delete your files  Since it looks like you re only letting the user input a plain dictionary  use ast literal eval    It safely does what you want and nothing more

User · Answer

If all you need is a user provided dictionary  possible better solution is json loads   The main limitation is that json dicts requires string keys   Also you can only provide literal data  but that is also the case for literal eval

User · Answer

eval  This is very powerful  but is also very dangerous if you accept strings to evaluate from untrusted input  Suppose the string being evaluated is  os system  rm -rf        It will really start deleting all the files on your computer  ast literal eval  Safely evaluate an expression node or a string containing a Python literal or container display  The string or node provided may only consist of the following Python literal structures  strings  bytes  numbers  tuples  lists  dicts  sets  booleans  None  bytes and sets  Syntax   eval expression  globals None  locals None  import ast ast literal eval node or string    Example     python 2 x - doesn t accept operators in string format import ast ast literal eval   1  2  3       output   1  2  3  ast literal eval  1 1     output  ValueError  malformed string     python 3 0 -3 6 import ast ast literal eval  1 1     output   2 ast literal eval    a   2   b   3  3  xyz       output     a   2   b   3  3  xyz     type dictionary ast literal eval          output   Syntax Error required only one parameter ast literal eval    import    os   system  rm -rf         output   error  eval    import    os   system  rm -rf          output   start deleting all the files on your computer    restricting using global and local variables eval    import    os   system  rm -rf          builtins              output   Error due to blocked imports by passing     builtins       in global    But still eval is not safe  we can access and break the code as given below s        lambda fc   lambda n        c for c in               class     bases   0    subclasses              if c   name      n       0     fc  function        fc  code            0 0 0 0  KABOOM                 0                         eval s      builtins           In the above code      class     bases   0  nothing but object itself   Now we instantiated all the subclasses  here our main enter code hereobjective is to find one class named n from it   We need to code object and function object from instantiated subclasses  This is an alternative way from CPython to access subclasses of object and attach the system   From python 3 7 ast literal eval   is now stricter  Addition and subtraction of arbitrary numbers are no longer allowed  link

[python] Using python's eval() vs. ast.literal_eval()?

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