So I have a program that reads a CSV file and performs some computations with the data before outputting it to a separate file. It was working fine yesterday, but when I came back it the program terminates without ever calling the for loop that iterates through each line of the CSV. No error is given. Does anyone know the reason for this?
Below is the function.
def my_map(my_input_stream, my_output_stream, my_mapper_input_parameters):
#files = glob.glob(my_input_stream)
#f = open(my_input_stream,"w")
out = codecs.open(my_output_stream,"w")
with open(my_input_stream) as csv_file:
csv_reader = csv.reader(csv_file)
for line in csv_reader:
stop_station = line[7]
start_station = line[3]
print(start_station)
out.write(f"{start_station}\t(1,0)\n")
print("Writing to file..")
out.write(f"{stop_station}\t(0,1)\n")
out.close()
Everything works fine apart from the for loop.
One of the lines you commented out is
f = open(my_input_stream,"w")
If you ever ran this, then my_input_stream would have been opened in "write" mode. This may have overwritten the data in the file and left you with a blank file, which would explain why the for loop is "skipped" - there's no data to iterate over.
Can you verify that my_input_stream still has actual data in it?
Related
This question already has answers here:
Iterating on a file doesn't work the second time [duplicate]
(4 answers)
Closed 1 year ago.
I was refactoring some code for my program and I have a mistake somewhere in the process. I am reading and writing .csv files.
In the beginning of my program I iterate through a .csv file in order to find which data from the file I need.
with open(csvPath, mode='r') as inputFile:
csvReader = csv.reader(inputFile)
potentialVals = []
paramVals = {}
for row in csvReader:
if row[3] == "Parameter":
continue
# Increment vales in dict
if row[3] not in paramVals:
paramVals[row[3]] = 1
else:
paramVals[row[3]] += 1
This iterates and works fine, the for loop gets me every row in the .csv file. I them perform some calculations and go to iterate through the same .csv file again later, and then select data to write to a new .csv file. My problem is here, when I go to iterate through a second time, it only gives me the first row of the .csv file, and nothing else.
# Write all of the information to our new csv file
with open(outputPath, mode='w') as outputFile:
csvWriter = csv.writer(outputFile, delimiter=',', quotechar='"', quoting=csv.QUOTE_ALL)
inputFile.seek(0)
rowNum = 0
for row in csvReader:
print(row)
Where the print statement is, it only prints the first line of the .csv file, and then exits the for loop. I'm not really sure what is causing this. I thought it might have been the
inputFile.seek(0)
But even if I opened a 2nd reader, the problem persisted. This for loop was working before I refactored it, all the other code is the same except the for loop I'm having trouble with, here is what it used to look like:
Edit: So I thought maybe it was a variable instance error, so I tried renaming my variables instead of reusing them and the issue persisted. Going to try a new file instance now,
Edit 2: Okay so this is interesting, when I look at the line_num value for my reader object (when I open a new one instead of using .seek) it does output 1, so I am at the beginning of my file. And when I look at the len(list(csvReader)) it is 229703, which shows that the .csv is fully there, so still not sure why it won't do anything besides the first row of the .csv
Edit 3: Just as a hail mary attempt, I tried creating a deep copy of the .csv file and iterating through that, but same results. I also tried just doing an entire separate .csv file and I also got the same issue of only getting 1 row. I guess that eliminates that it's a file issue, the information is there but there is something preventing it from reading it.
Edit 4: Here is where I'm currently at with the same issue. I might just have to rewrite this method completely haha but I'm going to lunch so I won't be able to actively respond now. Thank you for the help so far though!
# TODO: BUG HERE
with open(csvPath, mode='r') as inputFile2:
csvReader2 = csv.reader(inputFile2)
...
for row2 in csvReader2:
print("CSV Line Num: " + str(csvReader2.line_num))
print("CSV Index: " + str(rowNum))
print("CSV Length: " + str(len(list(csvReader2))))
print("CSV Row: " + str(row2))
Also incase it helps, here is csvPath:
nameOfInput = input("Please enter the file you'd like to convert: ")
csvPath = os.path.dirname(os.path.realpath(nameOfInput))
csvPath = os.path.join(csvPath, nameOfInput)
If you read the documentation carefully, it says csv reader is just a parser and all the heavy lifting is done by the underlying file object.
In your case, you are trying to read from a closed file in the second iteration and that is why it isn't working.
For csv reader to work you'll need an underlying object which supports the iterator protocol and returns a string each time its next() method is called — file objects and list objects are both suitable.
Link to the documentation: https://docs.python.org/3/library/csv.html
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
Im writing a simple command line program involving id's, i want to make it so that every time i run a function it should add a value to a CSV file. I have made a function that i thought worked, but apparently not. One of my for loops is not running correctly ;( Here is my code:
def addId(id):
file = open(ID_FILE_PATH, 'wb+')
read = csv.reader(file)
write = csv.writer(file)
existingRows = []
existingRows.append(id)
for rows in read: # This does not run
print rows
if len(rows) > 0 and rows[0] not in existingRows:
existingRows.append(rows[0])
for rows in existingRows: # This runs
write.writerow([rows])
file.close()
Sorry for my bad English btw.
You open the file with:
file = open(ID_FILE_PATH, 'wb+')
According to the documentation:
note that 'w+' truncates the file
You truncate the file, so no wonder there is nothing to read in! Use rb+ instead.
Opening an exiting file and writing a dictionary to it sometimes appends the dictionary (being written as a row) to the end of the previous line. How would I setup this code to always prevent that? It seems to only happen when a previous code 'run' has crashed and I have restarted the code and it begins to write to a file from a previous run (which I want - just not added onto the end of a previous line).
#Creating output dictwriter for results
with open(csv, 'a', 0) as outputFile:
fieldnames = csvCols
successWriter = csv.DictWriter(outputFile, fieldnames=fieldnames)
successWriter.writerow(out_dict)
outputFile.close()
Would opening the file in 'ab' mode make a difference?
You would usually add "\n" to the file line so that it writes to the next line.
The following should work fine in Python 2.x:
# Creating output dictwriter for results
with open(csv, 'ab') as outputFile:
successWriter = csv.DictWriter(outputFile, fieldnames=csvCols)
successWriter.writerow(out_dict)
The with statement will automatically close your file, even if there is a problem with your script. If you do not use the ab mode, it will result in an extra empty row per call of writerow.
If you are using Python 3.x, you would need the following:
# Creating output dictwriter for results
with open(csv, 'a', newline='') as outputFile:
successWriter = csv.DictWriter(outputFile, fieldnames=csvCols)
successWriter.writerow(out_dict)
print("writing text to file")
prompt = '>'
data = [input(prompt) for i in range(3)]
with open('textfile.txt', 'w') as testfile:
testfile.write("\n".join(data))
with open('textfile.txt', 'r') as testfile:
print (testfile.read())
data = [line.strip('\n') for line in testfile]
data2 = testfile.readlines()
print(data)
print(data2)
After learning how to read and write from text files I have been trying to use
for line in textfile
But to no avail. In my above code both data and data2 print as empty arrays which makes me think I am doing something really wrong. Before I could get testfile.readlines() to work but I was never able to use a for loop. For some reason it wouldn't even enter the loop (even if I do a standard for loop outside of list comprehension).
Does anyone have any ideas what I am doing incorrectly? I could not find anyone else who has this problem.
When you called
print (testfile.read())
That put the file pointer to the end of the file. You need to bring it back to the beginning again by calling
testfile.seek(0)
After that, so that when the next file reading method is called it will be able to read the file from the beginning again. Likewise, after that list comprehension assignment to data you will need to do the same so that data2 can be populated.
The first thing you do is print(testfile.read()) which reads the entire contents of the file. After that any read is going to fail. You need to seek back to the beginning of the file:
testfile.seek(0)