Is there an easy way to tell which line number a file pointer is on?

user_327026 0
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In Python 2.5, I am reading a structured text data file (~30 MB in size) using a file pointer:

fp = open('myfile.txt', 'r')
line = fp.readline()
# ... many other fp.readline() processing steps, which
# are used in different contexts to read the structures

But then, while parsing the file, I hit something interesting that I want to report the line number of, so I can investigate the file in a text editor. I can use fp.tell() to tell me where the byte offset is (e.g. 16548974L), but there is no "fp.tell_line_number()" to help me translate this to a line number.

Is there either a Python built-in or extension to easily track and "tell" what line number a text file pointer is on?

Note: I'm not asking to use a line_number += 1 style counter, as I call fp.readline() in different contexts and that approach would require more debugging than it is worth to insert the counter in the right corners of the code.

text-files line-numbers python
9 Answers

The following code will print the line number(where the pointer is currently on) while traversing through the file('testfile')

file=open("testfile", "r")
for line_no, line in enumerate(file):
    print line_no     # The content of the line is in variable 'line'
file.close()

output:

1
2
3
...
user_45083100

Messing around with a similar problem recently and came up with this class based solution.

class TextFileProcessor(object):

    def __init__(self, path_to_file):
        self.print_line_mod_number = 0
        self.__path_to_file = path_to_file
        self.__line_number = 0

    def __printLineNumberMod(self):
        if self.print_line_mod_number != 0:
            if self.__line_number % self.print_line_mod_number == 0:
                print(self.__line_number)

    def processFile(self):
        with open(self.__path_to_file, 'r', encoding='utf-8') as text_file:
            for self.__line_number, line in enumerate(text_file, start=1):
                self.__printLineNumberMod()

                # do some stuff with line here.

Set the print_line_mod_number property to the cadence you want logged and then call processFile.

For example... if you want feedback every 100 lines it would look like this.

tfp = TextFileProcessor('C:\\myfile.txt')
tfp.print_line_mod_number = 100
tfp.processFile()

The console output would be 

100
200
300
400
etc...
user_251970

Open file using with context manager and enumerate lines in a for loop.

with open('file_name.ext', 'r') as f:
    [(line_num, line) for line_num, line in enumerate(f)]
user_87826720
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