谁能帮我解释这个python代码吗？

Question

7.2 Write a program that prompts for a file name, then opens that file and reads through the file, looking for lines of the form:

  X-DSPAM-Confidence: 0.8475 

Count these lines and extract the floating point values from each of the lines and compute the average of those values and produce an output as shown below.

You can download the sample data at http://www.pythonlearn.com/code/mbox-short.txt when you are testing below enter mbox-short.txt as the file name.

This is what I did and got the output but is it the right way to do it?:

fname = raw_input("Enter file name: ")
fh = open(fname)
sum = 0
count = 0
for line in fh:
if not line.startswith("X-DSPAM-Confidence:") : continue
    line = line.replace("X-DSPAM-Confidence:","")
    line = line.strip()
    sum = sum + float(line)
    count = count + 1

avg = sum/count

print "Average spam confidence:",float(avg)

However, in few other sides, this one, and GitHub the code has been written differently. Can anyone explain it to me?

1)

fname = raw_input("Enter file name: ")
fh = open(fname)
tot = 0.0
count = 0
for line in fh:
    if not line.startswith("X-DSPAM-Confidence:") : continue
    words = line.split()
    tot = tot + float(words[1])
    count = count + 1
print "Average spam confidence:", tot/count

What does split() do here? Remove blank spaces and print it on the next line right? So how does that help? And what will float(words[1]) be then? Sorry is this sounds silly but this is my 3rd day into Python and I'm just trying to learn.

2) Lots others have used sum this way:

 a = line.split(':')
     sum = sum + float(a[1])

How does this work?

Answer 1

split will split a single string into multiples as a list, using either blank space or whatever character you pass as the parameter. By taking [1] you're using the second of the resulting strings, since indexes start at 0.

float will ignore leading spaces, so if you split on : and it leaves some spaces it doesn't matter.

Answer 2

1. str.split() split a string into a list. Check the examples in the document for more deatils. By the way:

    >>> '1 2 3 4'.split() ['1', '2', '3', '4'] >>> '1:2:3:4'.split() ['1:2:3:4'] >>> '1:2:3:4'.split(':') ['1', '2', '3', '4'] >>> 'X-DSPAM-Confidence: 0.8475'.split() ['X-DSPAM-Confidence:', '0.8475'] >>> 'X-DSPAM-Confidence: 0.8475'.split(':') ['X-DSPAM-Confidence', ' 0.8475'] 

2. a[1] is list index in this case. As the above example:

    >>> a = 'X-DSPAM-Confidence: 0.8475'.split(':') >>> a[0] 'X-DSPAM-Confidence' >>> a[1] ' 0.8475' 

   It's:

    ['X-DSPAM-Confidence', ' 0.8475'] ^^^^ ^^^^ a[0] a[1] 

3. float(x) : Return a floating point number constructed from a number or string x .

   It's easy to understand:

    >>> float('0.8475') 0.8475 >>> a = 'X-DSPAM-Confidence: 0.8475'.split(':') >>> float(a[1]) + 0.5 1.3475 >>> a[1] + 0.5 Traceback (most recent call last): File "<input>", line 1, in <module> TypeError: Can't convert 'float' object to str implicitly 

   If you don't do the convert, Python will raise Type when you run + between string object and float object like the above example.

   And however, float() ignores spaces. So:

    >>> float(' 0.8475 ') 0.8475 

Answer 3

In your file input you have "X-DSPAM-Confidence" always appearing like - XDSPAM-Confidence: 0.6959 . So there is space due to which your line.split() splits the required line like this:

['X-DSPAM-Confidence:', '0.9846']

Since by default space is argument for split command.