How does exec work with locals?

by

This issue is somewhat discussed in the Python3 bug list. Ultimately, to get this behavior, you need to do:

def foo():
    ldict = {}
    exec("a=3",globals(),ldict)
    a = ldict['a']
    print(a)

And if you check the Python3 documentation on exec, you’ll see the following note:

The default locals act as described for function locals() below: modifications to the default locals dictionary should not be attempted. Pass an explicit locals dictionary if you need to see effects of the code on locals after function exec() returns.

That means that one-argument exec can’t safely perform any operations that would bind local variables, including variable assignment, imports, function definitions, class definitions, etc. It can assign to globals if it uses a global declaration, but not locals.

Referring back to a specific message on the bug report, Georg Brandl says:

To modify the locals of a function on the fly is not
possible without several consequences: normally, function locals are not
stored in a dictionary, but an array, whose indices are determined at
compile time from the known locales. This collides at least with new
locals added by exec. The old exec statement circumvented this, because
the compiler knew that if an exec without globals/locals args occurred
in a function, that namespace would be “unoptimized”, i.e. not using the
locals array. Since exec() is now a normal function, the compiler does
not know what “exec” may be bound to, and therefore can not treat is
specially.

Emphasis is mine.

So the gist of it is that Python3 can better optimize the use of local variables by not allowing this behavior by default.

And for the sake of completeness, as mentioned in the comments above, this does work as expected in Python 2.X:

Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) 
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def f():
...     a = 1
...     exec "a=3"
...     print a
... 
>>> f()
3

More Related Contents:

  1. Behavior of exec function in Python 2 and Python 3
  2. How to get local variables updated, when using the `exec` call?
  3. exec() not working inside function python3.x
  4. Searching for IP addresses in a file
  5. How to optimise my code efficiency to speed up large data file extraction programs?
  6. ImportError: No module named ‘encodings’
  7. Tkinter: “Python may not be configured for Tk”
  8. Could not find a version that satisfies the requirement tensorflow
  9. Python was not found; run without arguments to install from the Microsoft Store, or disable this shortcut from Settings
  10. Import Error: No module named numpy
  11. How to deal with Kivy installing error in Python 3.8?
  12. PermissionError: [WinError 32] The process cannot access the file because it is being used by another process
  13. what are all the dtypes that pandas recognizes?
  14. Removing control characters from a string in python
  15. Accessing dict_keys element by index in Python3
  16. Trying to parse `request.body` from POST in Django [duplicate]
  17. Lazy evaluation in Python
  18. Parse SGML with Open Arbitrary Tags in Python 3
  19. How can I access variables from the caller, even if it isn’t an enclosing scope (i.e., implement dynamic scoping)?
  20. In python, if a function doesn’t have a return statement, what does it return? [duplicate]
  21. Creating an Ordered Counter
  22. Will OrderedDict become redundant in Python 3.7?
  23. Sort a list of tuples depending on two elements [duplicate]
  24. pytest cannot find module [duplicate]
  25. Reading a file using a relative path in a Python project
  26. How to replicate array to specific length array
  27. How to use a variable as function name in Python
  28. match dates using python regular expressions
  29. How to get value of variable entered from user input?
  30. Add custom form fields that are not part of the model (Django)

Leave a Comment