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What is the purpose of readline().strip()?

What is the purpose of readline().strip() (especially in the below code)?
Context:
I was taking a look at the following code:
op = open('encyin.txt', 'r')
n, q = op.readline().split()
n = int(n)
q = int(q)
dic = {}
for i in range(1, n + 1):
dic[str(i)]=(op.readline().strip())
And trying to interpret it.
My Interpretation:
The start is simple enough - it opens a file encyin.txt in read mode. It takes input - n & p - from the line, the .split() separating the two inputs. They are then classified as integers, and an empty list dict is created?
From there, a for loop is utilised.
But what does the last line mean? I am not familiar with (a) readline().strip() and (b) how this affects list dict and the values of the input:
For Example
If ency.txt was the following:
6 5
1151
723
1321
815
780
931
What happens to the other numbers from the 2nd line downwards? Does the readline().split assign them a line number? Does it add it to the list dict, a bit like .append?
What does the last line mean of the top code do? I am not familiar with (a) readline().strip() and (b) how this affects list dict and the values of the input:

In your text file, you have these things called whitespace characters. Often, these are spaces or enters ('\n') that you want to get rid of. The strip() helps you remove these whitespace characters.
If you were to print the numbers after reading them and without stripping, you would get:
number1
number2
number3
...
Because you haven't removed the hidden 'enter' character.

When reading a python script and you come across some function that you don't know, your goal should first to be understand the function out of context, and then you can figure out what they are doing in context.
The first port of call for understanding builtin/standard library functions (as opposed to functions from some extra library) should be the python docs. When the docs fail you, move on to other sources (there are plenty).
In this case, you want to know what op.readline() does. Well, what is op? I would go to open, and see that it creates a file object, which tells you that the actual implementation used is in io. Here we can search the page for readline.
What do the docs have to say about readline?
Read and return one line from the stream.
Here, I would assume, since it's a text file, "a line from the stream" is a string object (but you could always open a python interpreter to check), and look up string.strip(), which says:
Return a copy of the string with the leading and trailing characters removed. The chars argument is a string specifying the set of characters to be removed. If omitted or None, the chars argument defaults to removing whitespace.
Now put them together. They call (op.readline().strip()).
We know op is a "file object" using io
io's readline reads a single line from the stream
some_string.strip() called without parameters removes all whitespace from the start and end of some_string
Although python uses duck-typing, objects still have types/behaviours and understanding code often involves knowing what kind of object you are dealing with at any point so you can look into how it should work.
For example, if you know something is a dictionary, but you don't know what a dictionary is, you should search the docs for some info and try to understand what it does out of context first.

op = open('encyin.txt', 'r')
n, q = op.readline().split()
n = int(n)
q = int(q)
dic = {}
for i in range(1, n + 1):
# Here you're creating a key-value pair using the str value of the loop variable
# i as the dictionary key i.e. key dic[str(i)] creates the key, and the value is
# op.readline().strip(). strip() is a str method that removes trailing characters.
# the default is to remove whitespace at the beginning and ends of the string.
# These spaces get trimmed off if the method is called
dic[str(i)]=(op.readline().strip())
https://docs.python.org/3/library/stdtypes.html?highlight=str#str.strip

readline() returning a single line as string from your file.
ex: for the given txt file info:
Danni Loss
Shani Amari
Michele favarotti
readline() will return the first line:
Danni Loss\n
then there is a use of strip() removes all empty chars from the start and end of the string, so you will get:
Danni Loss

.readline() reads a line from a file. The result includes a trailing '\n'.
.strip() removes all leading & trailing whitespace (e.g. the above-mentioned '\n') from a string.
Thus, the last line of code dic[str(i)]=(op.readline().strip()) does the following:
Reads line from the open file
Strips whitespace from the line
Stores the stripped line in the dictionary using the index (converted to string) as a key

Why is my function returning blank strings, but works as expected in the python console?

Apologies for the poor title, I don't know how else to describe my situation.
I wrote a small pattern matching function.
def substrings(input_str):
'''Generate all leading substrings of an input string'''
for i in range(len(input_str)):
return input_str[:i]
It should return a series of slices of a string. If the input was ABCD, it should output ABCD, ABC, AB and A.
When I tested this function in the python console (shown below), it behaves correctly and outputs all the expected strings.
for i in range(len(input_str)):
print(input_str[:i])
But when used in the body of my program its returning nothing at all. For example;
test1 = substrings('ABCD')
print(test1)
Outputs blank lines and I'm struggling to figure out why.

In the console, your loop first prints out a blank string when i==0. Then it continues to loop and print out each of the characters in the string.
In the function, you are returning up to the 0th element in the array, which is the same blank string the console printed on the first time through the loop.
To see better what is happening in the console, you might print the index too:
for i in range(len(input_str)):
print('{0} {1}'.format(i, input_str[:i]))

That's because the first thing your functions returns is empty string ''. So you are exiting loop after first iteration for i = 0 and your variable is empty string because of the fact that:
>>> s = 'ABCD'
>>> s[:0]
''

You are returning in a loop. So the return is the last statement that would execute in a function. That is when a return statement is reached, the control leaves the function. So in the very first iteration i=0, the control returns '' and exits the function irrespective of for-loop. In console the output is obtained because I'm console each line is interpreted one-by-one unlike the program being compiled at once. So console shows the output. Hope this answer helps you

The issue is occurring because you are trying to return information in a loop. When the return statement is called, it exits the function. What you should do is use the yield keyword. Try doing this
def substrings(input_str):
for i in range(len(input_str)):
yield input_str[:i]
# Then iterate through the function like so
function = substrings()
for i in function:
print(i)
Your code should be working fine now!

Python variables stored in text files not working, but should be

I've settled on a text-file based save system for my game, storing the values of required variables with keywords - for example, the password that tells the game which chapter to play. However, it appears to be malfunctioning, and I can't see why.
Before starting the game, we have:
if not os.file.isfile('TSGsave{0}.txt'.format(name)):
TSGsave=open('TSGsave{0}.txt'.format(name),'wt')
TSGsave.write('\nw5CT$n<jfW=-#J%4Ya5##')
TSGsave.close()
(the keyword used is a bunch of jibberish so that the user can't change it knowing what's going to happen). This adds w5CT$n<jfW=-#J%4Ya5## to the text file. We then have:
for i in range (len(lines)):
if 'w5CT$n<jfW' in lines[i]:
findpass=lines[i]
for i in range (len(findpass)):
if findpass[i]=='=':
cutfrom=i+1
password=findpass[cutfrom:len(findpass)]
to retrieve the variable (which can change, so it can't be written in as definite value). I know it works, because I added print (password) to the code and it returned -#J%4Ya5##. Then to start the corresponding chapter, the code is:
if password=='-#J%4Ya5##':
but it isn't starting the indented block. In the shell, the program ends and goes back to the >>> line.
If there is a way to fix this code, great - but another code to do the same thing would work just as well.

Your lines contain newlines, and these are being included. Strip these from the line:
findpass = lines[i].rstrip('\n')
Printing a value with a newline in it will simply add an extra black line after the print. Always use the repr() function to produce a Python representation of strings to see such characters:
>>> print '-#J%4Ya5##\n'
-#J%4Ya5##
>>> print repr('-#J%4Ya5##\n')
'-#J%4Ya5##\n'
Your parsing code is overly complicated; you can use str.split() or str.partition() to split your password from the line instead. You should just loop over the lines list directly rather than produce indices with range():
for line in lines:
if 'w5CT$n<jfW' in line:
password = line.partition('=')[2].rstrip('\n')

Questions with s.endswith - python3.4

please tell me why my code didn't work the way I thought it would in python 3.4:
r = input("file address")
file = open(r)
for line in file:
if line.endswith("d") is True:
print(line)
my file is a *.txt and contains the following lines:
This line ends with a
this line ends with b
this line ends with c
this line ends with d
I expected python to print out the last one for it has the "d" at the end. However, the program did nothing.
When I change the boolean to "false", the python prints out all the lines. So I guess python sees the boolean "line.endswith("d") is False in its nature. How is that?
Thanks

Your lines contains newlines, so the conditional never evaluates to true.
You should either s.strip() the newline before calling str.endswith() or check for s. endswith("d\n").

It does that because you're not stripping each line. You need to run line = line.strip() for each line which gets rid of the \n (backslash-n) which is what each line actually ends with. So your line: This line ends with a this line ends with b this line ends with c this line ends with d actually ends in \n (backslash-n) but you don't see it because python doesn't print it. (It prints a new line)!
Good Luck!

You just need to check if line.endswith("d"):, it will either evaluate to True or False you don't need to use if line.endswith("d") is True and use line.strip().endswith("d") to remove the \n.
The last char is a "\n" so line.endswith("d") will never be True

Python: How to ignore #comment lines when reading in a file

In Python, I have just read a line form a text file and I'd like to know how to code to ignore comments with a hash # at the beginning of the line.
I think it should be something like this:
for
if line !contain #
then ...process line
else end for loop
But I'm new to Python and I don't know the syntax

you can use startswith()
eg
for line in open("file"):
li=line.strip()
if not li.startswith("#"):
print line.rstrip()

I recommend you don't ignore the whole line when you see a # character; just ignore the rest of the line. You can do that easily with a string method function called partition:
with open("filename") as f:
for line in f:
line = line.partition('#')[0]
line = line.rstrip()
# ... do something with line ...
partition returns a tuple: everything before the partition string, the partition string, and everything after the partition string. So, by indexing with [0] we take just the part before the partition string.
EDIT:
If you are using a version of Python that doesn't have partition(), here is code you could use:
with open("filename") as f:
for line in f:
line = line.split('#', 1)[0]
line = line.rstrip()
# ... do something with line ...
This splits the string on a '#' character, then keeps everything before the split. The 1 argument makes the .split() method stop after a one split; since we are just grabbing the 0th substring (by indexing with [0]) you would get the same answer without the 1 argument, but this might be a little bit faster. (Simplified from my original code thanks to a comment from #gnr. My original code was messier for no good reason; thanks, #gnr.)
You could also just write your own version of partition(). Here is one called part():
def part(s, s_part):
i0 = s.find(s_part)
i1 = i0 + len(s_part)
return (s[:i0], s[i0:i1], s[i1:])
#dalle noted that '#' can appear inside a string. It's not that easy to handle this case correctly, so I just ignored it, but I should have said something.
If your input file has simple enough rules for quoted strings, this isn't hard. It would be hard if you accepted any legal Python quoted string, because there are single-quoted, double-quoted, multiline quotes with a backslash escaping the end-of-line, triple quoted strings (using either single or double quotes), and even raw strings! The only possible way to correctly handle all that would be a complicated state machine.
But if we limit ourselves to just a simple quoted string, we can handle it with a simple state machine. We can even allow a backslash-quoted double quote inside the string.
c_backslash = '\\'
c_dquote = '"'
c_comment = '#'
def chop_comment(line):
# a little state machine with two state varaibles:
in_quote = False # whether we are in a quoted string right now
backslash_escape = False # true if we just saw a backslash
for i, ch in enumerate(line):
if not in_quote and ch == c_comment:
# not in a quote, saw a '#', it's a comment. Chop it and return!
return line[:i]
elif backslash_escape:
# we must have just seen a backslash; reset that flag and continue
backslash_escape = False
elif in_quote and ch == c_backslash:
# we are in a quote and we see a backslash; escape next char
backslash_escape = True
elif ch == c_dquote:
in_quote = not in_quote
return line
I didn't really want to get this complicated in a question tagged "beginner" but this state machine is reasonably simple, and I hope it will be interesting.

I'm coming at this late, but the problem of handling shell style (or python style) # comments is a very common one.
I've been using some code almost everytime I read a text file.
Problem is that it doesn't handle quoted or escaped comments properly. But it works for simple cases and is easy.
for line in whatever:
line = line.split('#',1)[0].strip()
if not line:
continue
# process line
A more robust solution is to use shlex:
import shlex
for line in instream:
lex = shlex.shlex(line)
lex.whitespace = '' # if you want to strip newlines, use '\n'
line = ''.join(list(lex))
if not line:
continue
# process decommented line
This shlex approach not only handles quotes and escapes properly, it adds a lot of cool functionality (like the ability to have files source other files if you want). I haven't tested it for speed on large files, but it is zippy enough of small stuff.
The common case when you're also splitting each input line into fields (on whitespace) is even simpler:
import shlex
for line in instream:
fields = shlex.split(line, comments=True)
if not fields:
continue
# process list of fields

This is the shortest possible form:
for line in open(filename):
if line.startswith('#'):
continue
# PROCESS LINE HERE
The startswith() method on a string returns True if the string you call it on starts with the string you passed in.
While this is okay in some circumstances like shell scripts, it has two problems. First, it doesn't specify how to open the file. The default mode for opening a file is 'r', which means 'read the file in binary mode'. Since you're expecting a text file it is better to open it with 'rt'. Although this distinction is irrelevant on UNIX-like operating systems, it's important on Windows (and on pre-OS X Macs).
The second problem is the open file handle. The open() function returns a file object, and it's considered good practice to close files when you're done with them. To do that, call the close() method on the object. Now, Python will probably do this for you, eventually; in Python objects are reference-counted, and when an object's reference count goes to zero it gets freed, and at some point after an object is freed Python will call its destructor (a special method called __del__). Note that I said probably: Python has a bad habit of not actually calling the destructor on objects whose reference count drops to zero shortly before the program finishes. I guess it's in a hurry!
For short-lived programs like shell scripts, and particularly for file objects, this doesn't matter. Your operating system will automatically clean up any file handles left open when the program finishes. But if you opened the file, read the contents, then started a long computation without explicitly closing the file handle first, Python is likely to leave the file handle open during your computation. And that's bad practice.
This version will work in any 2.x version of Python, and fixes both the problems I discussed above:
f = open(file, 'rt')
for line in f:
if line.startswith('#'):
continue
# PROCESS LINE HERE
f.close()
This is the best general form for older versions of Python.
As suggested by steveha, using the "with" statement is now considered best practice. If you're using 2.6 or above you should write it this way:
with open(filename, 'rt') as f:
for line in f:
if line.startswith('#'):
continue
# PROCESS LINE HERE
The "with" statement will clean up the file handle for you.
In your question you said "lines that start with #", so that's what I've shown you here. If you want to filter out lines that start with optional whitespace and then a '#', you should strip the whitespace before looking for the '#'. In that case, you should change this:
if line.startswith('#'):
to this:
if line.lstrip().startswith('#'):
In Python, strings are immutable, so this doesn't change the value of line. The lstrip() method returns a copy of the string with all its leading whitespace removed.

I've found recently that a generator function does a great job of this. I've used similar functions to skip comment lines, blank lines, etc.
I define my function as
def skip_comments(file):
for line in file:
if not line.strip().startswith('#'):
yield line
That way, I can just do
f = open('testfile')
for line in skip_comments(f):
print line
This is reusable across all my code, and I can add any additional handling/logging/etc. that I need.

I know that this is an old thread, but this is a generator function that I
use for my own purposes. It strips comments no matter where they
appear in the line, as well as stripping leading/trailing whitespace and
blank lines. The following source text:
# Comment line 1
# Comment line 2
# host01 # This host commented out.
host02 # This host not commented out.
host03
host04 # Oops! Included leading whitespace in error!
will yield:
host02
host03
host04
Here is documented code, which includes a demo:
def strip_comments(item, *, token='#'):
"""Generator. Strips comments and whitespace from input lines.
This generator strips comments, leading/trailing whitespace, and
blank lines from its input.
Arguments:
item (obj): Object to strip comments from.
token (str, optional): Comment delimiter. Defaults to ``#``.
Yields:
str: Next uncommented non-blank line from ``item`` with
comments and leading/trailing whitespace stripped.
"""
for line in item:
s = line.split(token, 1)[0].strip()
if s:
yield s
if __name__ == '__main__':
HOSTS = """# Comment line 1
# Comment line 2
# host01 # This host commented out.
host02 # This host not commented out.
host03
host04 # Oops! Included leading whitespace in error!""".split('\n')
hosts = strip_comments(HOSTS)
print('\n'.join(h for h in hosts))
The normal use case will be to strip the comments from a file (i.e., a hosts file, as in my example above). If this is the case, then the tail end of the above code would be modified to:
if __name__ == '__main__':
with open('aa.txt', 'r') as f:
hosts = strip_comments(f)
for host in hosts:
print('\'%s\'' % host)

A more compact version of a filtering expression can also look like this:
for line in (l for l in open(filename) if not l.startswith('#')):
# do something with line
(l for ... ) is called "generator expression" which acts here as a wrapping iterator that will filter out all unneeded lines from file while iterating over it. Don't confuse it with the same thing in square brakets [l for ... ] which is a "list comprehension" that will first read all the lines from the file into memory and only then will start iterating over it.
Sometimes you might want to have it less one-liney and more readable:
lines = open(filename)
lines = (l for l in lines if ... )
# more filters and mappings you might want
for line in lines:
# do something with line
All the filters will be executed on the fly in one iteration.

Use regex re.compile("^(?:\s+)*#|(?:\s+)") to skip the new lines and comments.

I tend to use
for line in lines:
if '#' not in line:
#do something
This will ignore the whole line, though the answer which includes rpartition has my upvote as it can include any information from before the #

a good thing to get rid of coments that works for both inline and on a line
def clear_coments(f):
new_text = ''
for line in f.readlines():
if "#" in line: line = line.split("#")[0]
new_text += line
return new_text