[Python学习25] 关于函数更多的

在这一章的学习中，做了一些函数和变量的练习。并不是直接运行脚本，而是在脚本中定义了一些函数，把他们导入到Python中通过执行函数的方式运行。先看代码：

def break_words(stuff):
    """This function will break up words for us."""
    words = stuff.split(' ')
    return words

def sort_words(words):
    """Sorts the words."""
    return sorted(words)

def print_first_word(words):
    """Prints the first word after popping it off."""
    word = words.pop(0)
    print word

def print_last_word(words):
    """Prints the last word after popping it off."""
    word = words.pop(-1)
    print word

def sort_sentence(sentence):
    """Takes in a full sentence and returns the sorted words."""
    words = break_words(sentence)
    return sort_words(words)

def print_first_and_last(sentence):
    """Prints the first and last words of the sentence."""
    words = break_words(sentence)
    print_first_word(words)
    print_last_word(words)

def print_first_and_last_sorted(sentence):
    """Sorts the words then prints the first and last one."""
    words = sort_sentence(sentence)
    print_first_word(words)
    print_last_word(words)

可以看到这个程序中只定义了函数，并没有调用函数并打印出来。我们需要使用import的方法把整个程序导入到python中，然后直接在python中使用程序中的各种功能。
导入函数的方法有两种：import no25 或 from no25 import * (我写的脚本名称叫no25.py）
下面是执行结果：

-userdeMacBook-Air:desktop user$ python
Python 2.7.11 (v2.7.11:6d1b6a68f775, Dec  5 2015, 12:54:16) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import no25
>>> sentence = "I am the king of the world!"
>>> word = no25.break_words(sentence)
>>> word
['I', 'am', 'the', 'king', 'of', 'the', 'world!']
>>> sorted_word = no25.sort_words(word)
>>> sorted_word
['I', 'am', 'king', 'of', 'the', 'the', 'world!']
>>> no25.print_first_word(word)
I
>>> no25.print_last_word(word)
world!
['am', 'the', 'king', 'of', 'the']
>>> no25.print_first_word(word)
am
>>> no25.print_last_word(word)
the
>>> sorted_sentence = no25.sort_sentence(sentence)
>>> sorted_sentence
['I', 'am', 'king', 'of', 'the', 'the', 'world!']
>>> no25.print_first_and_last(sentence)
I
world!
>>> no25.print_first_and_last_sorted(sentence)
I
world!
>>> 

-userdeMacBook-Air:Desktop user$ python
Python 2.7.11 (v2.7.11:6d1b6a68f775, Dec  5 2015, 12:54:16) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from no25 import *
>>> sentence = "All things was very good!"
>>> short = break_words(sentence)
>>> short
['All', 'things', 'was', 'very', 'good!']
>>> sort_words(short)
['All', 'good!', 'things', 'very', 'was']
>>> print_first_word(short)
All
>>> print_last_word(short)
good!
>>> sorted_sentence = sort_sentence(sentence)
>>> short
['things', 'was', 'very']
>>> sorted_words = sort_sentence(sentence)
>>> sorted_words
['All', 'good!', 'things', 'very', 'was']
>>> print_first_and_last(sentence)
All
good!
>>> print_first_and_last_sorted(sentence)
All
was
>>> 

下面是在Python中执行时遇到的一些错误：

错误1：split方法中引号里没有添加空格。
'split'方法中必须指定一个分隔符，如果引号中没有任何内容，就会提示“语法错误”，"ValueError: empty separator"。
正确用法：使用split(' ')，分隔符，默认为所有的空字符，包括空格、换行(\n)、制表符(\t)等。

Bing到这个网页，在StackOverflow上有人有同样的问题。
https://stackoverflow.com/questions/20826788/str-split-giving-me-valueerror-empty-separator-for-a-sentence-in-the-for

可惜我的网络一直打不开网页，只能通过Bing的Cache来访问，555……
http://cncc.bingj.com/cache.aspx?q=ValueError%3a+empty+separator&d=5000365845973185&mkt=en-US&setlang=en-US&w=WwhQN8t8C5Lh52BiYEvwWPh7GKnyWyYu

Eldad AK这位老兄解释的很清楚

具体错误提示如下：

-userdeMacBook-Air:desktop user$ python
Python 2.7.11 (v2.7.11:6d1b6a68f775, Dec  5 2015, 12:54:16) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> import sys
>>> import no25
>>> s = "There are many books."
>>> sen = no25.break_words(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "no25.py", line 4, in break_words
    words = stuff.split('')
 ValueError: empty separator

错误2：调用函数打错字导致python提示名称未定义。
我在程序中第46行下面调用了一个函数print_first_words()，但是在程序里并没有定义这个函数，而是有print_first_word()这个函数，所以是手误打错了，python的错误提示"NameError: …… is not defined"，可以帮助我们快速定位问题。

>>> no25.print_first_and_last(sentence)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "no25.py", line 46, in print_first_and_last
   print_first_words(words)
NameError: global name 'print_first_words' is not defined

错误3：当前目录没有no25.py脚本，我的脚本放在Desktop下，而新开的mac Command Line的目录为当前用户的Home目录。
可以看到python提示"No module named no25"，说明python在库中找不到叫no25的模块，仔细观察一下，发现我使用的是相对路径，当前目录是～，也就是user用户的家目录，所以找不到。

-userdeMacBook-Air:~ user$ python
Python 2.7.11 (v2.7.11:6d1b6a68f775, Dec  5 2015, 12:54:16) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from no25 import *
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named no25
>>> import no25
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named no25