pprint.isrecursive: Check if object requires recursive representation

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- By Vasudev Ram - Online Python training / SQL training / Linux training

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Tree image attribution

Hi, readers,

I was using the pprint module to pretty-print some Python data structures in a program I was writing. Then saw that it has a function called isrecursive.

The docstring for pprint.isrecursive says:

>>> print pprint.isrecursive.__doc__
Determine if object requires a recursive representation.

Here is a Python 3 shell session that shows what the isrecursive function does, with a list:

>>> import pprint
>>> print(pprint.pprint.__doc__)
Pretty-print a Python object to a stream [default is sys.stdout].
>>> a = []
>>> a
[]
>>> pprint.isrecursive(a)
False
>>>
>>> a.append(a)
>>> a
[[...]]
>>>
>>> pprint.isrecursive(a)
True

How about for a dict?

>>> b = {}
>>> pprint.isrecursive(b)
False
>>>
>>> b[1] = b
>>> b
{1: {...}}
>>> id(b) == id(b[1])
True
>>> pprint.isrecursive(b)
True

How about if an object is recursive, but not directly, like in the above two examples? Instead, it is recursive via a chain of objects:

>>> c = []
>>> d = []
>>> e = []
>>> c.append(d)
>>> d.append(e)
>>> c
[[[]]]
>>> pprint.isrecursive(c)
False
>>> e.append(c)
>>> c
[[[[...]]]]
>>> pprint.isrecursive(c)
True

So we can see that isrecursive is useful to detect some recursive Python object structures.
Interestingly, if I compare c with c[0] (after making c a recursive structure), I get:

>>> c == c[0]
Traceback (most recent call last):
  File "", line 1, in 
RecursionError: maximum recursion depth exceeded in comparison
>>>

In Python 2, I get:

RuntimeError: maximum recursion depth exceeded in cmp

Also, relevant XKCD-clone.

The image at the top of the post is of a tree created in the LOGO programming language using recursion.

- Enjoy.

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- Vasudev Ram - Online Python training and consulting

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I conduct online courses on Python programming, Unix / Linux commands and shell scripting and SQL programming and database design, with course material and personal coaching sessions.

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