How can I pretty print a nltk tree object?

Question

I want to view if the result below is what I need in a visual way:

import nltk 
sentence = [("the", "DT"), ("little", "JJ"), ("yellow", "JJ"), ("dog", "NN"), ("barked","VBD"), ("at", "IN"), ("the", "DT"), ("cat", "NN")]

pattern = """NP: {<DT>?<JJ>*<NN>}
VBD: {<VBD>}
IN: {<IN>}"""
NPChunker = nltk.RegexpParser(pattern) 
result = NPChunker.parse(sentence)

source: https://stackoverflow.com/a/31937278/3552975

I don't why I cannot pretty_print the result.

result.pretty_print()

The error reads that TypeError: not all arguments converted during string formatting. I use Python3.5, nltk3.3.

Answer 1

If you're looking for a bracketed parse output, you can use Tree.pprint():

>>> import nltk 
>>> sentence = [("the", "DT"), ("little", "JJ"), ("yellow", "JJ"), ("dog", "NN"), ("barked","VBD"), ("at", "IN"), ("the", "DT"), ("cat", "NN")]
>>> 
>>> pattern = """NP: {<DT>?<JJ>*<NN>}
... VBD: {<VBD>}
... IN: {<IN>}"""
>>> NPChunker = nltk.RegexpParser(pattern) 
>>> result = NPChunker.parse(sentence)
>>> result.pprint()
(S
  (NP the/DT little/JJ yellow/JJ dog/NN)
  (VBD barked/VBD)
  (IN at/IN)
  (NP the/DT cat/NN))

But most probably you're looking for

                             S                                      
            _________________|_____________________________          
           NP                        VBD       IN          NP       
   ________|_________________         |        |      _____|____     
the/DT little/JJ yellow/JJ dog/NN barked/VBD at/IN the/DT     cat/NN

Lets dig into the code from the Tree.pretty_print() https://github.com/nltk/nltk/blob/develop/nltk/tree.py#L692 :

def pretty_print(self, sentence=None, highlight=(), stream=None, **kwargs):
    """
    Pretty-print this tree as ASCII or Unicode art.
    For explanation of the arguments, see the documentation for
    `nltk.treeprettyprinter.TreePrettyPrinter`.
    """
    from nltk.treeprettyprinter import TreePrettyPrinter
    print(TreePrettyPrinter(self, sentence, highlight).text(**kwargs),
          file=stream)

It's creating a TreePrettyPrinter object, https://github.com/nltk/nltk/blob/develop/nltk/treeprettyprinter.py#L50

class TreePrettyPrinter(object):
    def __init__(self, tree, sentence=None, highlight=()):
        if sentence is None:
            leaves = tree.leaves()
            if (leaves and not any(len(a) == 0 for a in tree.subtrees())
                    and all(isinstance(a, int) for a in leaves)):
                sentence = [str(a) for a in leaves]
            else:
                # this deals with empty nodes (frontier non-terminals)
                # and multiple/mixed terminals under non-terminals.
                tree = tree.copy(True)
                sentence = []
                for a in tree.subtrees():
                    if len(a) == 0:
                        a.append(len(sentence))
                        sentence.append(None)
                    elif any(not isinstance(b, Tree) for b in a):
                        for n, b in enumerate(a):
                            if not isinstance(b, Tree):
                                a[n] = len(sentence)
                                sentence.append('%s' % b)
        self.nodes, self.coords, self.edges, self.highlight = self.nodecoords(
                tree, sentence, highlight)

And it looks like the line raising the error is sentence.append('%s' % b) https://github.com/nltk/nltk/blob/develop/nltk/treeprettyprinter.py#L97

Question is why did it raise a TypeError?

TypeError: not all arguments converted during string formatting

If we look carefully, it looks let we can use print('%s' % b) for most basic python types

# String
>>> x = 'abc'
>>> type(x)
<class 'str'>
>>> print('%s' % x)
abc

# Integer
>>> x = 123
>>> type(x)
<class 'int'>
>>> print('%s' % x)
123

# Float 
>>> x = 1.23
>>> type(x)
<class 'float'>
>>> print('%s' % x)
1.23

# Boolean
>>> x = True
>>> type(x)
<class 'bool'>
>>> print('%s' % x)
True

Surprisingly, it even works on list!

>>> x = ['abc', 'def']
>>> type(x)
<class 'list'>
>>> print('%s' % x)
['abc', 'def']

But it got stymied by tuple!!

>>> x = ('DT', 123)
>>> x = ('abc', 'def')
>>> type(x)
<class 'tuple'>
>>> print('%s' % x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: not all arguments converted during string formatting

So if we go back to the code at https://github.com/nltk/nltk/blob/develop/nltk/treeprettyprinter.py#L95

if not isinstance(b, Tree):
    a[n] = len(sentence)
    sentence.append('%s' % b)

Since we know that sentence.append('%s' % b) can't handle tuple, adding a check for tuple type and concatenating items in the tuple somehow and converting into a str will produce the nice pretty_print:

if not isinstance(b, Tree):
    a[n] = len(sentence)
    if type(b) == tuple:
        b = '/'.join(b)
    sentence.append('%s' % b)

[out]:

                             S                                      
            _________________|_____________________________          
           NP                        VBD       IN          NP       
   ________|_________________         |        |      _____|____     
the/DT little/JJ yellow/JJ dog/NN barked/VBD at/IN the/DT     cat/NN

Without changing the nltk code, is it possible to still get the pretty print?

Lets look at how the result i.e. a Tree object looks like:

Tree('S', [Tree('NP', [('the', 'DT'), ('little', 'JJ'), ('yellow', 'JJ'), ('dog', 'NN')]), Tree('VBD', [('barked', 'VBD')]), Tree('IN', [('at', 'IN')]), Tree('NP', [('the', 'DT'), ('cat', 'NN')])])

It looks like the leaves are kept as list of tuples of string, e.g. [('the', 'DT'), ('cat', 'NN')], so we could do some hack such that it becomes list of string, e.g. [('the/DT'), ('cat/NN')], so that Tree.pretty_print() will play nice.

Since we know that Tree.pprint() helps use concatenate the tuples of strings to the form we want, i.e.

(S
  (NP the/DT little/JJ yellow/JJ dog/NN)
  (VBD barked/VBD)
  (IN at/IN)
  (NP the/DT cat/NN))

We can simply output to a bracketed parse string, then re-read the parse Tree object with Tree.fromstring():

from nltk import Tree
Tree.fromstring(str(result)).pretty_print()

Finalment:

import nltk 
sentence = [("the", "DT"), ("little", "JJ"), ("yellow", "JJ"), ("dog", "NN"), ("barked","VBD"), ("at", "IN"), ("the", "DT"), ("cat", "NN")]

pattern = """NP: {<DT>?<JJ>*<NN>}
VBD: {<VBD>}
IN: {<IN>}"""
NPChunker = nltk.RegexpParser(pattern) 
result = NPChunker.parse(sentence)

Tree.fromstring(str(result)).pretty_print()

[out]:

                             S                                      
            _________________|_____________________________          
           NP                        VBD       IN          NP       
   ________|_________________         |        |      _____|____     
the/DT little/JJ yellow/JJ dog/NN barked/VBD at/IN the/DT     cat/NN