[issue42259] pprint: infinite recursion for saferepr() when using nested objects, but str() works

[Irit Katriel]

Irit Katriel  added the comment:

The documentation change in the PR clarifies the current state of things. 

@serhiy.storchaka - do you have a view on whether we should aim for an actual 
fix, or just document and close?

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[issue42259] pprint: infinite recursion for saferepr() when using nested objects, but str() works

[Irit Katriel]

Change by Irit Katriel :


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nosy: +serhiy.storchaka

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[issue42259] pprint: infinite recursion for saferepr() when using nested objects, but str() works

[Andrei Kulakov]

Andrei Kulakov  added the comment:

The recursion protection in `saferepr` applies when two conditions are met: 

- the structure is subclassed from list, tuple or dict
- __repr__ is not overriden

In this case neither condition is met.

However, the recursion is caused by the `__repr__` so when it's removed, 
recursion doesn't happen (but not due to recursion protection).

Btw also note that recursive path must be continuous for recursion detection to 
apply, e.g. if it's list[cust_obj[list[cust_obj...]]], detection also won't 
work.

I don't think we can fix this in code in a straightforward way, because  we 
want to avoid recursively calling saferepr in case __repr__ does not recurse.

In other words, if we knew __repr__ DOES recurse, we could call saferepr 
recursively and apply recursion detection without any problems, but __repr__ 
might intentionally say something like "", and then 
recursively calling saferepr would be undesirable.

So unfortunately we lose the recursion detection because of that.

One possible option would be to add an optional param like *force_recursion*, 
to recurse with detection even on overridden *__repr__*. I'm not sure it's 
worth it. But that's something users can consider: subclass PrettyPrinter and 
override saferepr() and either remove the checks for __repr__ override or add a 
param to do just that.

Current docs really make it sound like any recursion that shows up in repr() 
will be protected; it's really much more limited than that. Adding PR to 
clarify the limitations.

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versions: +Python 3.11 -Python 3.8

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[issue42259] pprint: infinite recursion for saferepr() when using nested objects, but str() works

[Andrei Kulakov]

Change by Andrei Kulakov :


--
keywords: +patch
nosy: +andrei.avk
nosy_count: 2.0 -> 3.0
pull_requests: +28477
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/30256

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[issue42259] pprint: infinite recursion for saferepr() when using nested objects, but str() works

[Irit Katriel]

Irit Katriel  added the comment:

I think this is a bug. There is recursion detection in pprint for dicts, lists 
and tuples, but it only applies when __repr__ has not been overridden in a 
subclass.

If you remove the __repr__ definition from NiceObject then str(s) works.

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nosy: +iritkatriel
versions: +Python 3.10, Python 3.9

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[issue42259] pprint: infinite recursion for saferepr() when using nested objects, but str() works

[Danylo Hlynskyi]

New submission from Danylo Hlynskyi :

First, here's an example using str():

```
class NiceObject:
def __str__(self):
return str(self.__dict__)
def __repr__(self):
return str(self)

s = NiceObject()
s.self = s
```
This outputs:
```
>>> s
{'self': {...}}
```

When I replace str() call with pprint.saferepr(), I end up in infinite 
recursion.
```
import pprint

class ReallyNiceObject:
def __str__(self):
return pprint.saferepr(self.__dict__)
def __repr__(self):
return str(self)
```

Same happens for pprint.pformat(), with depth set.

Is this expected behavior?

--
components: Library (Lib)
messages: 380315
nosy: danbst
priority: normal
severity: normal
status: open
title: pprint: infinite recursion for saferepr() when using nested objects, but 
str() works
type: behavior
versions: Python 3.8

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