How to Read a File Line-By-Line in Python

Reading a File Line by Line in Python

If the idea here is to understand how to read a file line by line then all you need to do is:

with open(filename, 'r') as f:
  for line in f:
    print(line)

It's not typical to put this in a try-except block.

Coming back to your original code there are several mistakes there which I'm assuming stem from a lack of understanding of how classes are defined/work in python.

The way you've written that code suggests you perhaps come from a Java background. I highly recommend doing one of the myriad free and really good online python courses offered on Coursera, or EdX.

Anyways, here's how I would do it using a class:

class ReadFile:
    def __init__(self, path):
        self.path = path

    def print_data(self):
        with open(self.path, 'r') as f:
            for line in f:
                print(line)

if __name__ == "__main__":
    reader = ReadFile("H:\\Desktop\\TheFile.txt")
    reader.print_data()

read a text file line by line and check for a substring on 2 of the lines

The variable line does not magically get updated when you call f.next() (or next(f) in Python 3). You would instead have to assign the line returned by next to a variable and test against it:

with open(my_file,'r') as f:
    for line in f:
        if 'text1' in line:
            try:
                next_line = next(f)
            except StopIteration:
                break # in case we are already at the end of file
            if 'text2' in next_line:
                # do some processing with line and next_line

If there can be cases where there are two consecutive lines with text1 followed by a line of text2, however, calling next(f) would make it consume the second line of text1, making it unable to match text1 in the next iteration. To remedy that, you can use the pairwise recipe in the itertools documentation to iterate 2 lines at a time over the file instead:

import itertools

def pairwise(iterable):
    a, b = itertools.tee(iterable)
    next(b, None)
    return zip(a, b)

with open(my_file,'r') as f:
    for line, next_line in pairwise(f):
        if 'text1' in line and 'text2' in next_line:
            # do some processing with line and next_line

Reading line by line from a file in python

You need to iterate through the file rather than the line:

#! /usr/bin/python
file = open('/home/results/err.txt')
for line in file:
    print line

file.readline() only reads the first line. When you iterate over it, you are iterating over the characters in the line.

How to read a text file line by line like a set of commands

Hey I just wrote the code on repl.it you should check it out. But here is the breakdown.

Read each line from file

Now you should a list where each element is a new line from the file

lines = ["command argument", "command argument" ... "command argument"]

Now iterate through each element in the list where you split the element at the " " (space character) and append it to a new list where all the commands and their respective arguments will be stored.
```
for line in lines:
    commands.append(line.split(" "))
```

Now the commands list should be a multidimensional array containing data like

commands = [["command", "argument"], ["command", "argument"], ... ["command", "argument"]]

Now you can just iterate through each sub-list where value at index 0 is the command and value at index 1 is the argument. After this you can use if statements to check for what command/ function to run with what datatype as an argument

HERE IS THE WHOLE CODE:

    command = []
    with open("command_files.txt", "r") as f:
        lines = f.read().strip().split("\n") # removing spaces on both ends, and spliting at the new line character \n
        print(lines) # now we have a list where each element is a line from the file
        # breaking each line at " " (space) to get command and the argument
        for line in lines:
            # appending the list to command list
            command.append(line.split(" "))
       # now the command list should be a multidimensional array
       # we just have to go through each of the sub list and where the value at 0 index should be the command, and at index 1 the arguments
       for i in command:
           if i[0] == "print":
               print(i[1])
           else:
               print("Command not recognized")

How to read a large file - line by line?

The correct, fully Pythonic way to read a file is the following:

with open(...) as f:
    for line in f:
        # Do something with 'line'

The with statement handles opening and closing the file, including if an exception is raised in the inner block. The for line in f treats the file object f as an iterable, which automatically uses buffered I/O and memory management so you don't have to worry about large files.

There should be one -- and preferably only one -- obvious way to do it.

how do i read a text file in line-by-line using Python when the formatting is specific

You can store all of the points from line 3 and beyond in a list of lists.

You just need to replace:

# parse remaining lines
.
.
.
# having trouble here...
.
.
.

with:

line = list()
points = list()

for i in range(2,len(data)):
    line.extend(data[i].split())

points = [line[x:x+3] for x in range(0, len(line),3)]

or if you want to store each of them as separate lists, you can do the following:

x = list()
y = list()
pen = list()

for i in range(2,len(data)):
    line = data[i].split()
    for j in range(len(line)):
        if j%3 == 0:
            x.append(line[j])
        elif j%3 == 1:
            y.append(line[j])
        else:
            pen.append(line[j])

You can make plots easily this way.

python read from text file line by line

This script loads the file into dataset list:

dataset = []
with open(filename, 'r') as f_in:
    for items in f_in:
        dataset.append(items.split())

print(dataset)

Prints:

[['a', 'b', 'c'], ['b', 'c'], ['a', 'b', 'c'], ['d'], ['b', 'c']]