将.startswith（）用于字符串中的特定位置的问题

Question

I have a text file, which has many lines of data in it. I need to check each line of this text file and process the data contained within the line accordingly (ie save to a separate, tabulated .txt for analysis)

The text file is in the following format:

• Number 1 or 0 (denoting relevance of data)
• An ID for each line (referring to what the data is)
• The data itself (contained in rest of line)

So this is what two example lines may look like:

1 ID:K-95 list of data

0 ID:D-56 list of other data

Such that the first line had relevant data to ID K-95 and the second had irrelevant data to ID D-56.

I want to parse the text file, and sort the data contained within each line based on the relevance (0 or 1) and the data ID. Ie save each line with the same ID in order of relevance (first all the lines with 1 and then with 0). Lines can have the same ID, but different data. Lines are also always of a fixed length.

To do this I came up with:

idtag = input('Enter ID:')

with open("example.txt", 'r') as f:                                                                                         
    for line in f.readlines():                                                                                              
        if line.startswith('1') and line.startswith(idtag, 5, 3):                                                                                            
            print line

Having trouble with this however. Specifically around the second condition after the and operator. I can print/select lines based on whether there is a 0 or 1, no problem. However, using the .startswith() method with a defined position seems to return nothing: no error, no printing - it simply executes and returns nothing.

Any ideas? Maybe a better way of parsing this data to meet my objective?

Answer 1

The start and end are interpreted as absolute positions (specifically: end is not interpreted relative to start ) for str.startswith :

str.startswith(prefix[, start[, end]])

Return True if string starts with the prefix , otherwise return False . prefix can also be a tuple of prefixes to look for. With optional start , test string beginning at that position. With optional end , stop comparing string at that position.

So instead of

line.startswith(idtag, 5, 3)

you need to use

line.startswith(idtag, 5, 5+4)

The two parameters are equivalent to slicing notation:

line[5: 5+4].startswith(idtag)

For example:

>>> a = 'abcdefg'
>>> a.startswith('c', 2, 1)
False
>>> a[2:1]
''

>>> a.startswith('c', 2)
True
>>> a[2:]
'cdefg'

>>> a.startswith('c', 2, 3)
True
>>> a[2:3]
'c'

Answer 2

I realise there's already an answer, but as an alternative you could also just check if idtag exists in the line:

idtag = input('Enter ID:')

with open("example.txt", 'r') as f:                                                                                         
    for line in f.readlines():                                                                                              
        if line.startswith('1') and idtag in line:                                                                                            
            print line