eval fails in list comprehension [duplicate]

by

By using list comprehension, you actually define a new scope. Indeed if we alter the list comprehension to:

out2 = [print(globals()) or print(locals()) or eval(cmd) for cmd in ['self.b']]

we force Python to print the local and global variables before making the eval(..) call, and we obtain something like:

{'__builtins__': <module 'builtins' (built-in)>, '__name__': '__main__', '__loader__': <class '_frozen_importlib.BuiltinImporter'>, 'b': <__main__.B object at 0x7f406f55d4a8>, '__doc__': None, '__package__': None, 'B': <class '__main__.B'>, '__spec__': None}
{'.0': <list_iterator object at 0x7f406f55df28>, 'cmd': 'self.b'}

So as local variables we only have a .0 and a cmd.

You can however pass self to the list comprehension by using:

globs = globals()
locs = locals()
out2 = [eval(cmd,globs,locs) for cmd in ['self.b']]

So now eval(..) will use the local and global variables as defined in the scope of the function. Since we use locs and globs. Python will pass references to these dictionaries to the eval(..) call.

Finally a warning as with every use of eval(..): eval is a dangerous function. You better use it only if you really need it.

An additional side effect of this additional scope (introduced in python-3.x) is that the loop variable does not leak: after the list comprehension cmd is cleaned up: you can no longer access it (usually it would hold the last element it has handled). For example:

>>> [x for x in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined

More Related Contents:

  1. how to create a list of lists using list comprehension [closed]
  2. How to create the following lists with list comprehensions? [closed]
  3. Accessing class variables from a list comprehension in the class definition
  4. Why is a list comprehension so much faster than appending to a list?
  5. List comprehension and lambdas in Python [duplicate]
  6. Add an element in each dictionary of a list (list comprehension)
  7. Are list comprehensions syntactic sugar for `list(generator expression)` in Python 3?
  8. Appending to a list comprehension in Python returns None [duplicate]
  9. Converting python string into bytes directly without eval()
  10. List comprehension scope error from Python debugger
  11. Why is Python’s eval() rejecting this multiline string, and how can I fix it?
  12. Skip elements on a condition based in a list comprehension in python
  13. Is doing multiplication multiple times or assigning to a variable faster?
  14. How do I create a scoring system in Python? [closed]
  15. How to end a python program without it going to the next line of code [duplicate]
  16. What is an alternative to execfile in Python 3?
  17. Behavior of exec function in Python 2 and Python 3
  18. Remove and Replace Printed items [duplicate]
  19. jumping too fast?
  20. How to get local variables updated, when using the `exec` call?
  21. How to activate virtualenv in Linux?
  22. How to fix AttributeError: partially initialized module?
  23. How to use values stored in variables as case patterns?
  24. Floor division with negative number
  25. Python and Windows Named Pipes
  26. Resolving metaclass conflicts
  27. What’s the best way to store a phone number in Django models?
  28. How to fix “could not find or load the Qt platform plugin windows” while using Matplotlib in PyCharm
  29. asyncio.ensure_future vs. BaseEventLoop.create_task vs. simple coroutine?
  30. Python: How to read and load an excel file from AWS S3?

Leave a Comment