How do I execute a string containing Python code in Python

Question

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The most logical solution would be to use the built-in eval   function  Another solution is to write that string to a temporary python file and execute it

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For statements  use exec string   Python 2 3  or exec string  Python 2     gt  gt  gt  mycode    print  hello world    gt  gt  gt  exec mycode  Hello world   When you need the value of an expression  use eval string     gt  gt  gt  x   eval  2 2    gt  gt  gt  x 4   However  the first step should be to ask yourself if you really need to   Executing code should generally be the position of last resort  It s slow  ugly and dangerous if it can contain user-entered code   You should always look at alternatives first  such as higher order functions  to see if these can better meet your needs

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I tried quite a few things  but the only thing that work was the following   temp dict      exec  temp dict  val     10    print temp dict  val      output   10

User · Answer

Check out eval   x   1 print eval  x 1   - gt 2

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Use eval

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eval and exec are the correct solution  and they can be used in a safer manner   As discussed in Python s reference manual and clearly explained in this tutorial  the eval and exec functions take two extra parameters that allow a user to specify what global and local functions and variables are available   For example   public variable   10  private variable   2  def public function        return  public information   def private function        return  super sensitive information     make a list of safe functions safe list     public variable    public function   safe dict   dict    k  locals   get k  None   for k in safe list      add any needed builtins back in safe dict  len     len   gt  gt  gt  eval  public variable 2       builtins      None    safe dict  12   gt  gt  gt  eval  private variable 2       builtins      None    safe dict  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt string gt    line 1  in  lt module gt  NameError  name  private variable  is not defined   gt  gt  gt  exec  print     s  has  i characters      public function    len public function           builtins      None   safe dict   public information  has 18 characters   gt  gt  gt  exec  print     s  has  i characters      private function    len private function           builtins      None   safe dict  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt string gt    line 1  in  lt module gt  NameError  name  private function  is not defined   In essence you are defining the namespace in which the code will be executed

User · Answer

eval   is just for expressions  while eval  x 1   works  eval  x 1   won t work for example  In that case  it s better to use exec  or even better  try to find a better solution

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It s worth mentioning  that  exec s brother exist as well called execfile if you want to call a python file  That is sometimes good if you are working in a third party package which have terrible IDE s included and you want to code outside of their package    Example   execfile   path to source py    or    exec open   path to source py   read

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In the example a string is executed as code using the exec function   import sys import StringIO    create file-like string to capture output codeOut   StringIO StringIO   codeErr   StringIO StringIO    code       def f x       x   x   1     return x  print  This is my output          capture output and errors sys stdout   codeOut sys stderr   codeErr  exec code    restore stdout and stderr sys stdout   sys   stdout   sys stderr   sys   stderr    print f 4   s   codeErr getvalue    print  error  n s n    s  s   codeOut getvalue    print  output  n s    s  codeOut close   codeErr close

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Remember that from version 3 exec is a function  so always use exec mystring  instead of exec mystring

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Ok    I know this isn t exactly an answer  but possibly a note for people looking at this as I was  I wanted to execute specific code for different users customers but also wanted to avoid the exec eval  I initially looked to storing the code in a database for each user and doing the above   I ended up creating the files on the file system within a  customer filters  folder and using the  imp  module  if no filter applied for that customer  it just carried on  import imp   def get customer module customerName  default   name  filter        lm   None     try          module name   customerName     name          m   imp find module module name    customer filters            lm   imp load module module name  m 0   m 1   m 2       except                      ignore  if no module is found       return lm  m   get customer module customerName   filter   if m is not None      m apply address filter myobj    so customerName    jj  would execute apply address filter from the customer filters jj filter py file

User · Answer

As the others mentioned  it s  exec       but  in case your code contains variables  you can use  global  to access it  also to prevent the compiler to raise the following error      NameError  name  p variable  is not defined   exec  p variable    1 2 3 4    global p variable print p variable

User · Answer

Avoid exec and eval Using exec and eval in Python is highly frowned upon  There are better alternatives From the top answer  emphasis mine    For statements  use exec  When you need the value of an expression  use eval  However  the first step should be to ask yourself if you really need to  Executing code should generally be the position of last resort  It s slow  ugly and dangerous if it can contain user-entered code  You should always look at alternatives first  such as higher order functions  to see if these can better meet your needs   From Alternatives to exec eval   set and get values of variables with the names in strings  while eval  would work  it is generally not advised to use variable names bearing a meaning to the program itself  Instead  better use a dict   It is not idiomatic From http   lucumr pocoo org 2011 2 1 exec-in-python   emphasis mine   Python is not PHP Don t try to circumvent Python idioms because some other language does it differently  Namespaces are in Python for a reason and just because it gives you the tool exec it does not mean you should use that tool   It is dangerous From http   nedbatchelder com blog 201206 eval really is dangerous html  emphasis mine   So eval is not safe  even if you remove all the globals and the builtins  The problem with all of these attempts to protect eval   is that they are blacklists  They explicitly remove things that could be dangerous  That is a losing battle because if there s just one item left off the list  you can attack the system  So  can eval be made safe  Hard to say  At this point  my best guess is that you can t do any harm if you can t use any double underscores  so maybe if you exclude any string with double underscores you are safe  Maybe     It is hard to read and understand From http   stupidpythonideas blogspot it 2013 05 why-evalexec-is-bad html  emphasis mine    First  exec makes it harder to human beings to read your code  In order to figure out what s happening  I don t just have to read your code  I have to read your code  figure out what string it s going to generate  then read that virtual code  So  if you re working on a team  or publishing open source software  or asking for help somewhere like StackOverflow  you re making it harder for other people to help you  And if there s any chance that you re going to be debugging or expanding on this code 6 months from now  you re making it harder for yourself directly

User · Answer

You accomplish executing code using exec  as with the following IDLE session    gt  gt  gt  kw       gt  gt  gt  exec   ret   4    in kw  gt  gt  gt  kw  ret    4

[python] How do I execute a string containing Python code in Python?

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