Cannot import Python class

Question

I am using Python 2.7, and trying to walk through the study guide Learn Python The Hard Way. I am having a problem when I try Exercise 49.

Python 2.7.10 (default, Jul 14 2015, 19:46:27) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>> from ex48.parser import *
>>> x = parse_sentence([('verb', 'run'), ('direction', 'north')])
>>> x.subject
'player'
>>> x.verb
'run'
>>> x.object
'north'
>>> x = parse_sentence([('noun', 'bear'), ('verb', 'eat'), ('stop', 'the'), ('noun', 'honey')])
>>> x.subject
'bear'
>>> x.verb
'eat'
>>> x.object
'honey'

When I do:

from ex48.parser import *
x = parse_sentence([('verb', 'run'), ('direction', 'north')])

Windows PowerShell gives me:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'parse_sentence' is not defined

My file structure is:

../lexicon
     bin/
     docs/
     ex48/
         __init__.py
         lexicon.py
         parser.py
     setup.py
     tests/
         lexicon_tests.py
         __init__.py

Here is code in parser.py

class ParserError(Exception):
    pass

class Sentence(object):
    def __init__(self, subject, verb, obj):
        #remember we take ('noun', 'princess') tuples and convert them
        self.subject = subject[1]
        self.verb = verb[1]
        self.object = obj[1]

    def peek(word_list):
        if word_list:
            word = word_list[0]
            return word[0]
        else:
            return None

    def match(word_list, expecting):
        if word_list:
            word = word_list.pop(0)

            if word[0] == expecting:
                return word
            else:
                return None
        else:
            return None

    def skip(word_list, word_type):
        while peek(word_list) == word_type:
            match(word_list, word_type)

    def parse_verb(word_list):
        skip(word_list, 'stop')

        if peek(word_list) == 'verb':
            return match(word_list, 'verb')
        else:
            raise ParserError("Expected a verb next")

    def parse_object(word_list):
        skip(word_list, 'stop')
        next_word = peek(word_list)

        if next_word == 'noun':
            return match(word_list, 'noun')
        elif next_word == 'direction':
            return match(word_list, 'direction')
        else:
            raise ParserError("Expected a noun or direction next.")

    def parse_subject(word_list):
        skip(word_list, 'stop')
        next_word = peek(word_list)

        if next_word == 'noun':
            return match(word_list, 'noun')
        elif next_word == 'verb':
            return ('noun', 'player')
        else:
            raise ParserError("Expected a verb next.")

    def parse_sentence(word_list):
        subj = parse_subject(word_list)
        verb = parse_verb(word_list)
        obj = parse_object(word_list)

        return Sentence(subj, verb, obj)

It should work, assuming that parse_sentence is defined in parser.py. Can you show what that file contains? — Vlad
– Vlad, Commented Dec 29, 2015 at 19:13
It depends both on the contents of the file parser.py and your current directory. Ensure that you're running Python from the directory called lexicon. — steinar
– steinar, Commented Dec 29, 2015 at 19:17
@steinar, I tried again to make sure I run Python under lexicon directory, not any sub-directories. No luck so far — Gabriel.S
– Gabriel.S, Commented Dec 29, 2015 at 19:42

Ali Nikneshan · Accepted Answer · 2015-12-29 19:53:09Z

Bad Ident. only first function in part of class.

class ParserError(Exception):
    pass

class Sentence(object):
    def __init__(self, subject, verb, obj):
        #remember we take ('noun', 'princess') tuples and convert them
        self.subject = subject[1]
        self.verb = verb[1]
        self.object = obj[1]

def peek(word_list):
    if word_list:
        word = word_list[0]
        return word[0]
    else:
        return None

def match(word_list, expecting):
    if word_list:
        word = word_list.pop(0)

        if word[0] == expecting:
            return word
        else:
            return None
    else:
        return None

def skip(word_list, word_type):
    while peek(word_list) == word_type:
        match(word_list, word_type)

def parse_verb(word_list):
    skip(word_list, 'stop')

    if peek(word_list) == 'verb':
        return match(word_list, 'verb')
    else:
        raise ParserError("Expected a verb next")

def parse_object(word_list):
    skip(word_list, 'stop')
    next_word = peek(word_list)

    if next_word == 'noun':
        return match(word_list, 'noun')
    elif next_word == 'direction':
        return match(word_list, 'direction')
    else:
        raise ParserError("Expected a noun or direction next.")

def parse_subject(word_list):
    skip(word_list, 'stop')
    next_word = peek(word_list)

    if next_word == 'noun':
        return match(word_list, 'noun')
    elif next_word == 'verb':
        return ('noun', 'player')
    else:
        raise ParserError("Expected a verb next.")

def parse_sentence(word_list):
    subj = parse_subject(word_list)
    verb = parse_verb(word_list)
    obj = parse_object(word_list)

    return Sentence(subj, verb, obj)

It is work as keeping some data together. like struct in c

Vlad · Accepted Answer · 2015-12-29 19:47:01Z

0

parse_sentence() appears to be a method on Sentence. Either you want to unindent it so it's a top-level function in the file, or make it, for example, a @staticmethod and call it as Sentence.parse_sentence().

answered Dec 29, 2015 at 19:47

Vlad

18.8k4 gold badges44 silver badges72 bronze badges

2 Comments

Gabriel.S Over a year ago

Thanks @Vlad, I found that all the methods should be on Sentence except __init__(). Unindent them works, but what is the point of Class Sentence()?

Vlad Over a year ago

I think it's meant to group together the results, so you can access results like x.verb rather than e.g. x[1] if they had returned a tuple directly (e.g. return (subj, verb, obj)) in parser.

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