Issue 13272: 2to3 fix_renames doesn't rename string.lowercase/uppercase/letters

$ cat deleteme.py 
from string import lowercase, uppercase, letters
print uppercase == 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
print lowercase == 'abcdefghijklmnopqrstuvwxyz'
print letters == 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
$ python deleteme.py 
True
True
True
$ 2to3 -w deleteme.py 
RefactoringTool: Skipping implicit fixer: buffer
RefactoringTool: Skipping implicit fixer: idioms
RefactoringTool: Skipping implicit fixer: set_literal
RefactoringTool: Skipping implicit fixer: ws_comma
RefactoringTool: Refactored deleteme.py
--- deleteme.py (original)
+++ deleteme.py (refactored)
@@ -1,4 +1,4 @@
 from string import lowercase, uppercase, letters
-print uppercase == 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
-print lowercase == 'abcdefghijklmnopqrstuvwxyz'
-print letters == 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
+print(uppercase == 'ABCDEFGHIJKLMNOPQRSTUVWXYZ')
+print(lowercase == 'abcdefghijklmnopqrstuvwxyz')
+print(letters == 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ')
RefactoringTool: Files that were modified:
RefactoringTool: deleteme.py
$ python3 deleteme.py 
Traceback (most recent call last):
  File "deleteme.py", line 1, in <module>
    from string import lowercase, uppercase, letters
ImportError: cannot import name lowercase

They should be renamed to ascii_*.

@Ezio can you prepare a patch for this?

Changing the imports only is straightforward, but I'm having trouble detecting and changing future uses of the variables, without also clobbering user-defined variables with the same names.

I notice some of the current fixers have similar problems, for example the itertools fixer changes:


```
def imap(): pass
imap()
```

to
 

```
def imap(): pass
map()
```

This patch is a little smarter than that, but it only detects top-level definitions.  For example, the lowercase in 


```
class Foo:
  def __init__(self):
    self.lowercase = "blargh"
```

still gets renamed.

I can work on this more, but would like to know:

a) whether this fix is still wanted
b) how smart it needs to be
c) whether my approach is reasonable.  I feel like there should be an easier way.

...

d) how to format code :\

* there were many small issues with the patch, improved as good as possible the FixString to catch ```import as```, ```from string import *``` and ```import string```

* did not include the FixStringImports as it's not a necessity to work in Python3

* indeed, some uses of the constant names that are user-defined will also rename, only when it's in a ```import string```. Added a ```Caution``` in the documentation. Still, when all are changed, it should not affect the Python3 version, will just work with the renamed variable name.

nosy: + superluser
messages: + msg226659

keywords: + patch

messages: + msg221808
versions: + Python 3.4, Python 3.5, - Python 3.2, Python 3.3