python为什么不将stdin输入作为字典读取？

Question

I'm sure I'm doing something dumb here, but here goes. I'm working on a class assignment for my Udacity class "Intro to Map Reduce and Hadoop". Our assignment is to make a mapper/reducer that will count occurrences of a word across our data set (the body of forum posts). I've got an idea of how to do this, but I can't get Python to read in stdin data to the reducer as a dictionary.

Here's my approach thus far: Mapper reads through the data (in this case in the code) and spits out a dictionary of word:count for each forum post:

#!/usr/bin/python
import sys
import csv
import re
from collections import Counter


def mapper():
    reader = csv.reader(sys.stdin, delimiter='\t')
    writer = csv.writer(sys.stdout, delimiter='\t', quotechar='"', quoting=csv.QUOTE_ALL)

    for line in reader:
        body = line[4]
        #Counter(body)
        words = re.findall(r'\w+', body.lower())
        c = Counter(words)
        #print c.items()
        print dict(c)





test_text = """\"\"\t\"\"\t\"\"\t\"\"\t\"This is one sentence sentence\"\t\"\"
\"\"\t\"\"\t\"\"\t\"\"\t\"Also one sentence!\"\t\"\"
\"\"\t\"\"\t\"\"\t\"\"\t\"Hey!\nTwo sentences!\"\t\"\"
\"\"\t\"\"\t\"\"\t\"\"\t\"One. Two! Three?\"\t\"\"
\"\"\t\"\"\t\"\"\t\"\"\t\"One Period. Two Sentences\"\t\"\"
\"\"\t\"\"\t\"\"\t\"\"\t\"Three\nlines, one sentence\n\"\t\"\"
"""

# This function allows you to test the mapper with the provided test string
def main():
    import StringIO
    sys.stdin = StringIO.StringIO(test_text)
    mapper()
    sys.stdin = sys.__stdin__

if __name__ == "__main__":
    main()

the count of forum post then goes to stdout like: {'this': 1, 'is': 1, 'one': 1, 'sentence': 2}

then the reducer should read in this stdin as a dictionary

#!/usr/bin/python
import sys
from collections import Counter, defaultdict
for line in sys.stdin.readlines():
    print dict(line)

but that fails, giving me this error message: ValueError: dictionary update sequence element #0 has length 1; 2 is required ValueError: dictionary update sequence element #0 has length 1; 2 is required

Which means (if I understand correctly) that it's reading in each line not as a dict, but as a text string. How can I get python to understand that input line is a dict? I've tried using Counter and defaultdict, but still had the same problem or had it read in each character as an element of list, which is also not what I want.

Ideally, I want the mapper to read in the dict of each line, then add the values of the next line, so after the second line the values are {'this':1,'is':1,'one':2,'sentence':3,'also':1} and so on.

Thanks, JR

Answer 1

Thanks to @keyser, the ast.literal_eval() method worked for me. Here's what I have now:

#!/usr/bin/python
import sys
from collections import Counter, defaultdict
import ast
lineDict = {}
c = Counter()
for line in sys.stdin.readlines():
    lineDict = ast.literal_eval(line)
    c.update(lineDict)
print c.most_common()