Python Dict and File

python

Dict Hash Table

Python的哈希表结构叫做字典。基本形式为key:value的键值对的集合，被大括号包围。string数字和turple都可以作为key，任何类型都可以作为value。可以使用in或者dict.get(key)来确认key是否在字典中。

## Can build up a dict by starting with the the empty dict {}
## and storing key/value pairs into the dict like this:
## dict[key] = value-for-that-key
dict = {}
dict['a'] = 'alpha'
dict['g'] = 'gamma'
dict['o'] = 'omega'
print dict  ## {'a': 'alpha', 'o': 'omega', 'g': 'gamma'}
print dict['a']     ## Simple lookup, returns 'alpha'
dict['a'] = 6       ## Put new key/value into dict
'a' in dict         ## True
## print dict['z']                  ## Throws KeyError
if 'z' in dict: print dict['z']     ## Avoid KeyError
print dict.get('z')  ## None (instead of KeyError)

for循环能遍历一个字典的所有的key，而key的顺序是任意的。dict.keys和dict.values返回所有的key或者value。还有items()，它返回一系列的(key, value) tuple，这是最有效的确认字典中所有的键值数据的方法。这些list都可以传递给sorted函数。

## By default, iterating over a dict iterates over its keys.
## Note that the keys are in a random order.
for key in dict: print key
## prints a g o
## Exactly the same as above
for key in dict.keys(): print key
## Get the .keys() list:
print dict.keys()  ## ['a', 'o', 'g']
## Likewise, there's a .values() list of values
print dict.values()  ## ['alpha', 'omega', 'gamma']
## Common case -- loop over the keys in sorted order,
## accessing each key/value
for key in sorted(dict.keys()):
print key, dict[key]
## .items() is the dict expressed as (key, value) tuples
print dict.items()  ##  [('a', 'alpha'), ('o', 'omega'), ('g', 'gamma')]
## This loop syntax accesses the whole dict by looping
## over the .items() tuple list, accessing one (key, value)
## pair on each iteration.
for k, v in dict.items(): print k, '>', v
## a > alpha    o > omega     g > gamma

有一种变体的iterkeys()， itervalues() ， iteritems()可以避免建造全部的list，这在数据量很大的时候常用。

Dict Formatting

%操作符方便的把字典中的value代替为字符串：

hash = {}
hash['word'] = 'garfield'
hash['count'] = 42
s = 'I want %(count)d copies of %(word)s' % hash  # %d for int, %s for string
# 'I want 42 copies of garfield'

Del

del操作符删除元素，如：

var = 6
del var  # var no more!
list = ['a', 'b', 'c', 'd']
del list[0]     ## Delete first element
del list[-2:]   ## Delete last two elements
print list      ## ['b']
dict = {'a':1, 'b':2, 'c':3}
del dict['b']   ## Delete 'b' entry
print dict      ## {'a':1, 'c':3}

Files

open()函数打开并且返回一个文件代号，这可以接下来用来读或者写操作。f = open('name','r')的含义是打开一个文件传递给变量f，准备进行读操作，可以用f.close()关闭。还可以使用'w'用来写，'a'用来添加。特殊的'rU'用来将不同的行尾符转化为'\n'，for用来遍历文件的每一行很有效，不过注意这只对text文件有效，对二进制文件不起作用。

# Echo the contents of a file
f = open('foo.txt', 'rU')
for line in f:   ## iterates over the lines of the file
print line,    ## trailing , so print does not add an end-of-line char
               ## since 'line' already includes the end-of line.
f.close()

每次读一行的操作可以避免使用过多的内存。f.readlines()method读整个文件加入内存，并且返回一个由每一行组成的list。而f.read()method读整个文件为一条字符串。
对于写操作来说，f.write()method是把数据写入一个打开的输出文件的最简单的方法。或者用print >> f, string来打印到屏幕。

Files Unicode

codecs模块提供对于对于读取Unicode文件的支持。

import codecs
f = codecs.open('foo.txt', 'rU', 'utf-8')
for line in f:
# here line is a *unicode* string