Develop Reference

Python reading nothing from file [duplicate]

I am a beginner of Python. I am trying now figuring out why the second 'for' loop doesn't work in the following script. I mean that I could only get the result of the first 'for' loop, but nothing from the second one. I copied and pasted my script and the data csv in the below.
It will be helpful if you tell me why it goes in this way and how to make the second 'for' loop work as well.
My SCRIPT:
import csv
file = "data.csv"
fh = open(file, 'rb')
read = csv.DictReader(fh)
for e in read:
print(e['a'])
for e in read:
print(e['b'])
"data.csv":
a,b,c
tree,bough,trunk
animal,leg,trunk
fish,fin,body

The csv reader is an iterator over the file. Once you go through it once, you read to the end of the file, so there is no more to read. If you need to go through it again, you can seek to the beginning of the file:
fh.seek(0)
This will reset the file to the beginning so you can read it again. Depending on the code, it may also be necessary to skip the field name header:
next(fh)
This is necessary for your code, since the DictReader consumed that line the first time around to determine the field names, and it's not going to do that again. It may not be necessary for other uses of csv.
If the file isn't too big and you need to do several things with the data, you could also just read the whole thing into a list:
data = list(read)
Then you can do what you want with data.

I have created small piece of function which doe take path of csv file read and return list of dict at once then you loop through list very easily,
def read_csv_data(path):
"""
Reads CSV from given path and Return list of dict with Mapping
"""
data = csv.reader(open(path))
# Read the column names from the first line of the file
fields = data.next()
data_lines = []
for row in data:
items = dict(zip(fields, row))
data_lines.append(items)
return data_lines
Regards

Searching a text file and grabbing all lines that do not include ## in python

I am trying to write a python script to read in a large text file from some modeling results, grab the useful data and save it as a new array. The text file is output in a way that has a ## starting each line that is not useful. I need a way to search through and grab all the lines that do not include the ##. I am used to using grep -v in this situation and piping to a file. I want to do it in python!
Thanks a lot.
-Tyler

I would use something like this:
fh = open(r"C:\Path\To\File.txt", "r")
raw_text = fh.readlines()
clean_text = []
for line in raw_text:
if not line.startswith("##"):
clean_text.append(line)
Or you could also clean the newline and carriage return non-printing characters at the same time with a small modification:
for line in raw_text:
if not line.startswith("##"):
clean_text.append(line.rstrip("\r\n"))
You would be left with a list object that contains one line of required text per element. You could split this into individual words using string.split() which would give you a nested list per original list element which you could easily index (assuming your text has whitespaces of course).
clean_text[4][7]
would return the 5th line, 8th word.
Hope this helps.
[Edit: corrected indentation in loop]

My suggestion would be to do the following:
listoflines = [ ]
with open(.txt, "r") as f: # .txt = file, "r" = read
for line in f:
if line[:2] != "##": #Read until the second character
listoflines.append(line)
print listoflines
If you're feeling brave, you can also do the following, CREDITS GO TO ALEX THORNTON:
listoflines = [l for l in f if not l.startswith('##')]
The other answer is great as well, especially teaching the .startswith function, but I think this is the more pythonic way and also has the advantage of automatically closing the file as soon as you're done with it.

How to read from a file using only readline()

The code i have right now is this
f = open(SINGLE_FILENAME)
lines = [i for i in f.readlines()]
but my proffessor demands that
You may use readline(). You may not use read(), readlines() or iterate over the open file using for.
any suggestions?
thanks

You could use a two-argument iter() version:
lines = iter(f.readline, "")
If you need a list of lines:
lines = list(lines)

First draft:
lines = []
with open(SINGLE_FILENAME) as f:
while True:
line = f.readline()
if line:
lines.append(line)
else:
break
I feel fairly certain there is a better way to do it, but that does avoid iterating with for, using read, or using readlines.
You could write a generator function to keep calling readline() until the file was empty, but that doesn't really seem like a large improvement here.

Write strings to another file

The Problem - Update:
I could get the script to print out but had a hard time trying to figure out a way to put the stdout into a file instead of on a screen. the below script worked on printing results to the screen. I posted the solution right after this code, scroll to the [ solution ] at the bottom.
First post:
I'm using Python 2.7.3. I am trying to extract the last words of a text file after the colon (:) and write them into another txt file. So far I am able to print the results on the screen and it works perfectly, but when I try to write the results to a new file it gives me str has no attribute write/writeline. Here it the code snippet:
# the txt file I'm trying to extract last words from and write strings into a file
#Hello:there:buddy
#How:areyou:doing
#I:amFine:thanks
#thats:good:I:guess
x = raw_input("Enter the full path + file name + file extension you wish to use: ")
def ripple(x):
with open(x) as file:
for line in file:
for word in line.split():
if ':' in word:
try:
print word.split(':')[-1]
except (IndexError):
pass
ripple(x)
The code above works perfectly when printing to the screen. However I have spent hours reading Python's documentation and can't seem to find a way to have the results written to a file. I know how to open a file and write to it with writeline, readline, etc, but it doesn't seem to work with strings.
Any suggestions on how to achieve this?
PS: I didn't add the code that caused the write error, because I figured this would be easier to look at.
End of First Post
The Solution - Update:
Managed to get python to extract and save it into another file with the code below.
The Code:
inputFile = open ('c:/folder/Thefile.txt', 'r')
outputFile = open ('c:/folder/ExtractedFile.txt', 'w')
tempStore = outputFile
for line in inputFile:
for word in line.split():
if ':' in word:
splitting = word.split(':')[-1]
tempStore.writelines(splitting +'\n')
print splitting
inputFile.close()
outputFile.close()
Update:
checkout droogans code over mine, it was more efficient.

Try this:
with open('workfile', 'w') as f:
f.write(word.split(':')[-1] + '\n')
If you really want to use the print method, you can:
from __future__ import print_function
print("hi there", file=f)
according to Correct way to write line to file in Python. You should add the __future__ import if you are using python 2, if you are using python 3 it's already there.

I think your question is good, and when you're done, you should head over to code review and get your code looked at for other things I've noticed:
# the txt file I'm trying to extract last words from and write strings into a file
#Hello:there:buddy
#How:areyou:doing
#I:amFine:thanks
#thats:good:I:guess
First off, thanks for putting example file contents at the top of your question.
x = raw_input("Enter the full path + file name + file extension you wish to use: ")
I don't think this part is neccessary. You can just create a better parameter for ripple than x. I think file_loc is a pretty standard one.
def ripple(x):
with open(x) as file:
With open, you are able to mark the operation happening to the file. I also like to name my file object according to its job. In other words, with open(file_loc, 'r') as r: reminds me that r.foo is going to be my file that is being read from.
for line in file:
for word in line.split():
if ':' in word:
First off, your for word in line.split() statement does nothing but put the "Hello:there:buddy" string into a list: ["Hello:there:buddy"]. A better idea would be to pass split an argument, which does more or less what you're trying to do here. For example, "Hello:there:buddy".split(":") would output ['Hello', 'there', 'buddy'], making your search for colons an accomplished task.
try:
print word.split(':')[-1]
except (IndexError):
pass
Another advantage is that you won't need to check for an IndexError, since you'll have, at least, an empty string, which when split, comes back as an empty string. In other words, it'll write nothing for that line.
ripple(x)
For ripple(x), you would instead call ripple('/home/user/sometext.txt').
So, try looking over this, and explore code review. There's a guy named Winston who does really awesome work with Python and self-described newbies. I always pick up new tricks from that guy.
Here is my take on it, re-written out:
import os #for renaming the output file
def ripple(file_loc='/typical/location/while/developing.txt'):
outfile = "output.".join(os.path.basename(file_loc).split('.'))
with open(outfile, 'w') as w:
lines = open(file_loc, 'r').readlines() #everything is one giant list
w.write('\n'.join([line.split(':')[-1] for line in lines]))
ripple()
Try breaking this down, line by line, and changing things around. It's pretty condensed, but once you pick up comprehensions and using lists, it'll be more natural to read code this way.

You are trying to call .write() on a string object.
You either got your arguments mixed up (you'll need to call fileobject.write(yourdata), not yourdata.write(fileobject)) or you accidentally re-used the same variable for both your open destination file object and storing a string.

Storing iteration values

I have this code:
file_open=open("/python32/doc1.txt","r")
file=a1.read().lower()
for line in file:
line_word=line.split()
This works fine. But if I print line_word it would be printed continuously.
I like to store in some variable, so that I may print a line of my choice and manipulate them at my choice.
Is there any way to solve this problem?
Generally I append to a list and do the necessary stuff, but for bigger files it is really problematic.
If anyone can kindly suggest a solution.

You don't need to call .read() to iterate over the lines of a file.
with open("/python32/doc1.txt", "r") as f:
for line in f:
line_word = line.lower().split()

Does this help?
file_open=open("/python32/doc1.txt","r")
file=a1.read().lower()
line_word = []
for line in file.splitlines():
line_word.append(line.split())