python - 试图理解一段非常简单的 python 代码 : return [x[0] for x in y] 的行为

Question

我认为(而且 Contino 仍然不明白为什么会有所不同)这段代码:

def categories(self):
    cur=self.con.execute('select category from cc');
    for d in cur:
        return d[0]

等同于这个:

def categories(self):
    cur=self.con.execute('select category from cc');
    return [d[0] for d in cur]

但是当我在我的代码中一个一个地替换另一个时，我在代码的其他地方得到了一个错误:

  File "C:\Users\CG\Desktop\Google Drive\Sci&Tech\projects\naivebayes\main.py", line 226, in post
    spam_result = nb.classify(given_sentence)
  File "C:\Users\CG\Desktop\Google Drive\Sci&Tech\projects\naivebayes\main.py", line 204, in classify
    if cat==best: continue
UnboundLocalError: local variable 'best' referenced before assignment

为什么会这样？为什么这两段代码不等价？

完整代码:

# -*- coding: utf-8 -*-
# -*- coding: utf-8 -*-

import sqlite3

import USSSALoader

import random

from pysqlite2 import dbapi2 as sqlite

import re

import math

def getfeatures(doc):
  splitter=re.compile('\\W*')
  # Split the words by non-alpha characters
  words=[s.lower() for s in splitter.split(doc)
          if len(s)>2 and len(s)<20]
  # Return the unique set of words only
#  return dict([(w,1) for w in words]).iteritems()
  return dict([(w,1) for w in words])


class classifier:
  def __init__(self,getfeatures, filename=None):
    # Counts of feature/category combinations
    self.fc={}
    # Counts of documents in each category
    self.cc={}
    self.getfeatures=getfeatures

#  def setdb(self,dbfile):
    self.con=sqlite.connect('db_file')
#    self.con=sqlite3.connect(":memory:")
    self.con.execute('create table if not exists fc(feature,category,count)')
    self.con.execute('create table if not exists cc(category,count)')

  def incf(self,f,cat):
    count=self.fcount(f,cat)
    if count==0:
      self.con.execute("insert into fc values ('%s','%s',1)" % (f,cat))
    else:
      self.con.execute(
        "update fc set count=%d where feature='%s' and category='%s'"
        % (count+1,f,cat))

  def fcount(self,f,cat):
    res=self.con.execute(
      'select count from fc where feature="%s" and category="%s"'
      %(f,cat)).fetchone()
    if res==None: return 0
    else: return float(res[0])

  def incc(self,cat):
    count=self.catcount(cat)
    if count==0:
      self.con.execute("insert into cc values ('%s',1)" % (cat))
    else:
      self.con.execute("update cc set count=%d where category='%s'"
                       % (count+1,cat))

  def catcount(self,cat):
    res=self.con.execute('select count from cc where category="%s"'
                         %(cat)).fetchone()
    if res==None: return 0
    else: return float(res[0])

  def categories(self):
    cur=self.con.execute('select category from cc');
#    return [d[0] for d in cur]
    for d in cur:
 #       print "d =", d
  #      print "d[0] =", d[0]
        return d[0]

  def totalcount(self):
    res=self.con.execute('select sum(count) from cc').fetchone();
    if res==None: return 0
    print "res=self.con.execute('select * FROM cc').fetchall(); = ", self.con.execute('select * FROM cc').fetchall();
    print 'res sum(count) = ', res
    print 'res[0] = ', res[0]
    return res[0]

  def train(self,item,cat):
    features=self.getfeatures(item)
    # Increment the count for every feature with this category
    for f in features:
##    for f in features:
      self.incf(f,cat)
    # Increment the count for this category
    self.incc(cat)
    self.con.commit()

  def fprob(self,f,cat):
    if self.catcount(cat)==0: return 0
    # The total number of times this feature appeared in this
    # category divided by the total number of items in this category
    return self.fcount(f,cat)/self.catcount(cat)

  def weightedprob(self,f,cat,prf,weight=1.0,ap=0.5):
    # Calculate current probability
    basicprob=prf(f,cat)
    # Count the number of times this feature has appeared in
    # all categories
    totals=sum([self.fcount(f,c) for c in self.categories()])
    # Calculate the weighted average
    bp=((weight*ap)+(totals*basicprob))/(weight+totals)
    return bp

class naivebayes(classifier):

  def __init__(self,getfeatures):
    classifier.__init__(self, getfeatures)
    self.thresholds={}

  def docprob(self,item,cat):
    features=self.getfeatures(item)
    # Multiply the probabilities of all the features together
    p=1
    for f in features: p*=self.weightedprob(f,cat,self.fprob)
    return p

  def prob(self,item,cat):
    catprob=self.catcount(cat)/self.totalcount()
    docprob=self.docprob(item,cat)
    return docprob*catprob

  def setthreshold(self,cat,t):
    self.thresholds[cat]=t

  def getthreshold(self,cat):
    if cat not in self.thresholds: return 1.0
    return self.thresholds[cat]

  def classify(self,item,default=None):
    probs={}
    # Find the category with the highest probability
    max=0.0
    for cat in self.categories():
      probs[cat]=self.prob(item,cat)
      if probs[cat]>max:
        max=probs[cat]
        best=cat

    # Make sure the probability exceeds threshold*next best
    for cat in probs:
      if cat==best: continue
      if probs[cat]*self.getthreshold(best)>probs[best]: return default
    return best

def sampletrain(cl):
  cl.train('Nobody owns the water.','good')
  cl.train('the quick rabbit jumps fences','good')
  cl.train('buy pharmaceuticals now','bad')
  cl.train('make quick money at the online casino','bad')
  cl.train('the quick brown fox jumps','good')


nb = naivebayes(getfeatures)

sampletrain(nb)


doc_test = "buy pharmaceuticals now or earn money at the online casino"

print ('\ndoc_test is classified as %s'%nb.classify(doc_test))

Answer 1

一个函数只返回一次。

当你看到

for d in cur:
    return d[0]

循环在第一次迭代期间返回。

但是这个list comprehension

return [d[0] for d in cur]

遍历 cur 中的每个项目以创建一个列表，然后返回结果。