I'm doing the following code that read all the line in a CSV file and after, it reads more information about a row chosen by the user.
This works only the second part print({rows[2]["Name"]})
with open("file.csv", "r", newline="") as a:
csv_reader = csv.DictReader(a)
rows = list(csv_reader)
for row in csv_reader: #to read all the line
x=csv_reader.line_num-3
print(x, row["Name"])
print({rows[2]["Name"]}) #to print the information about line 2
This works only the first part for row in csv_reader:
with open("file.csv", "r", newline="") as a:
csv_reader = csv.DictReader(a)
for row in csv_reader:
x=csv_reader.line_num-3
print(x, row["Name"])
rows = list(csv_reader) #THIS row changed position
print({rows[2]["Name"]})
And the second part has this error: IndexError: list index out of range
How can I solve it? the second part should work because the code alone works.
Related
What I want to do is actually as it is written in the title.
with open(path, "r+", newline='') as csv_file:
csv_reader = csv.reader(csv_file, delimiter=',')
list_of_column_names = []
num_cols = len(next(csv_reader))
for i in range(num_cols):
list_of_column_names.append(i)
fields = list_of_column_names
with open(example.csv, "r+", newline='') as writeFile:
csvwriter = csv.DictWriter(writeFile, delimiter=',', lineterminator='\n', fieldnames=fields)
writeFile.seek(0, 0)
csvwriter.writeheader()
I want to enumerate the columns which initially doesn't have any column names. But when I run the code, it replaces the data in the first row. For example:
example.csv:
a,b
c,d
e,f
what I want:
0,1
a,b
c,d
e,f
what happens after running the code:
0,1
c,d
e,f
Is there a way to prevent this from happening?
There's no magical way to insert a line into an existing text file.
The following is how I think of doing this, and your code is already getting steps 2-4. Also, I wouldn't mess with the DictWriter since you're not trying to convert a Python dict to CSV (I can see you using it for writing the header, but that's easy enough to do with the regular reader/writer):
open a new file for writing
read the first row of your CSV
interpret the column indexes as the header
write the header
write the first row
read/write the rest of the rows
move the new file back to the old file, overwrite (not shown)
Here's what that looks like in code:
import csv
with open('output.csv', 'w', newline='') as out_f:
writer = csv.writer(out_f)
with open('input.csv', newline='') as in_f:
reader = csv.reader(in_f)
# Read the first row
first_row = next(reader)
# Count the columns in first row; equivalent to your `for i in range(len(first_row)): ...`
header = [i for i, _ in enumerate(first_row)]
# Write header and first row
writer.writerow(header)
writer.writerow(first_row)
# Write rest of rows
for row in reader:
writer.writerow(row)
I have a following problem. Lets say I want to write a word on the cell in column = 1 & and row = 3.
I have written this function:
import csv
def write_to_csv(myfile, word):
with open(myfile, "w", newline="") as csv_file:
csv_writer = csv.writer(csv_file)
write = [word]
csv_writer.writerow(elt for elt in write)
write_to_csv("output.csv", "hello")
My function writes the word "hello" into the cell in column = 1 & and row = 1.
Now imagine that my output.csv already has someting in the first cell. And I don`t want to rewrite it. So how can I modify my function to write the word "hello" on column = 1 & and row = 3 ?
I found this queston, but it did not help me: How to select every Nth row in CSV file using python
Thank you very much!
A CSV file is a text file. That means that you should not try to overwrite it in place. The common way is to copy it to a new file introducing your changes at that time. When done, you move the new file with the old name.
Here is a possible code. It just hope that the file output.csv.tmp does not exist, but can be created and that output.csv has at least 4 lines:
def write_to_csv(myfile, word, row_nb, col_nb):
"""Updates a csv file by writing word at row_nb row and col_nb column"""
with open(myfile) as csv_file, open(myfile+'.tmp', "w", newline='') as out:
csv_reader = csv.reader(csv_file)
csv_writer = csv.writer(out)
#skip row_nb rows
for i in range(row_nb):
csv_writer.writerow(next(csv_reader))
# read and change the expected row
row = next(csv_reader)
row[col_nb] = word
# print(row) # uncomment for debugging
csv_writer.writerow(row)
# copy last rows
for row in csv_reader:
csv_writer.writerow(row)
# rename the tmp file
os.remove(myfile)
os.rename(myfile+'.tmp', myfile)
# write hello at first column of fourth row in output.csv
write_to_csv('output.csv', 'hello', 3, 0)
I have a csv file with 2 rows and multiple lines.
import csv
with open('data.csv', 'r') as csv_file:
csv_reader = csv.reader(csv_file)
next(csv_reader)
for row in csv_reader:
print(row[0])
The output is:
row0line0
row0line1
row0line2
...
Is there a way i could further separate the rows into a list of individual cells?
Thanks
As I understand your csv file look like this:
row0line0
row1line1
...
If its possible i should reccomand to change it to:
row0 line0
row1 line1
...
(Add a space between the rows and the lines)
Then you can update your code to the code bellow to print only the rows and create two lists - one that contain the rows and another that contain the lines:
import csv
with open('data.csv', 'r') as csv_file:
csv_reader = csv.reader(csv_file)
next(csv_reader)
rows = []
lines = []
for item in csv_reader:
temp = item[0].split(" ")
rows.append(temp[0])
lines.append(temp[1])
print(temp[0])
If you mean that each row becomes an element of the list that goes this way:
with open('data.csv', 'r') as csv_file:
reader = csv.reader(csv_file)
data_list = [row[0] for row in reader]
Otherwise, if you want to create a list of the first elements of each line, you can do this:
with open('data.csv', 'r') as csv_file:
reader = csv.reader(csv_file)
row0_list = []
for row in reader:
row0_list.append(row[0])
I hope the problem is solved with this explanation.
My understanding is that you are asking to output all the data fields. csv_reader is already separating your rows into a list individual cells!
You current script reads the file one line at a time and prints the first item in each row with this line:
for row in csv_reader:
print(row[0])
Instead of printing row[0], which only prints the first field in the csv row, you can just print the row:
for row in csv_reader:
print(row)
That will output the field lists (from my sample csv):
['r0v0', 'r0v1', 'r0v2']
['r1v0', 'r1v1', 'r1v2']
If you want to print in a nicer format, you can use join:
for row in csv_reader:
print(", ".join(row))
Output:
r0v0, r0v1, r0v2
r1v0, r1v1, r1v2
My csv:
r0v0,r0v1,r0v2
r1v0,r1v1,r1v2
I've just started coding, I'm trying to remove certain columns from a CSV for a project, we aren't supposed to use pandas. For instance, one of the fields I have to delete is called DwTm, but there's about 15 columns I have to get rid of; I only want the first few, Here's what I've gotten:
import csv
FTemp = "D:/tempfile.csv"
FOut = "D:/NewFile.csv"
with open(FTemp, 'r') as csv_file:
csv_reader = csv.reader(csv_file)
with open(FOut, 'w') as new_file:
fieldnames = ['Stn_Name', 'Lat', 'Long', 'Prov', 'Tm']
csv_writer = csv.DictWriter(new_file, fieldnames=fieldnames)
for line in csv_reader:
del line['DwTm']
csv_writer.writerow(line)
When I run this, I get the error
del line['DwTm']
TypeError: list indices must be integers or slices, not str
This is the only method I've found to almost work without using pandas. Any ideas?
The easiest way around this is to use a DictReader to read the file. Like DictWriter, which you are using to write the file, DictReader uses dictionaries for rows, so your approach of deleting keys from the old row then writing to the new file will work as you expect.
import csv
FTemp = "D:/tempfile.csv"
FOut = "D:/NewFile.csv"
with open(FTemp, 'r') as csv_file:
# Adjust the list to be have the correct order
old_fieldnames = ['Stn_Name', 'Lat', 'Long', 'Prov', 'Tm', 'DwTm']
csv_reader = csv.DictReader(csv_file, fieldnames=old_fieldnames)
with open(FOut, 'w') as new_file:
fieldnames = ['Stn_Name', 'Lat', 'Long', 'Prov', 'Tm']
csv_writer = csv.DictWriter(new_file, fieldnames=fieldnames)
for line in csv_reader:
del line['DwTm']
csv_writer.writerow(line)
Below
import csv
# We only want to read the 'department' field
# We are not interested in 'name' and 'birthday month'
# Make sure the list items are in ascending order
NON_INTERESTING_FIELDS_IDX = [2,0]
rows = []
with open('example.csv') as csv_file:
csv_reader = csv.reader(csv_file, delimiter=',')
for row in csv_reader:
for idx in NON_INTERESTING_FIELDS_IDX:
del row[idx]
rows.append(','.join(row))
with open('example_out.csv','w') as out:
for row in rows:
out.write(row + '\n')
example.csv
name,department,birthday month
John Smith,Accounting,November
Erica Meyers,IT,March
example_out.csv
department
Accounting
IT
It's possible to simultaneously open the file to read from and the file to write to. Let's say you know the indices of the columns you want to keep, say, 0,2, and 4:
good_cols = (0,2,4)
with open(Ftemp, 'r') as fin, open(Fout, 'w') as fout:
for line in fin:
line = line.rstrip() #clean up newlines
temp = line.split(',') #make a list from the line
data = [temp[x] for x in range(len(temp)) if x in good_cols]
fout.write(','.join(data) + '\n')
The list comprehension (data) pulls only the columns you want to keep out of each row and immediately writes line-by-line to your new file, using the join method (plus tacking on an endline for each new row).
If you only know the names of the fields you want to keep/remove it's a bit more involved, you have to extract the indices from the first line of the csv file, but it's not much more difficult.
I'm following some feedback from another thread, but have gotten stuck. I'm looking to search an existing csv file to locate the row in which a string occurs. I am then looking to update this row with new data.
What I have so far gives me an "TypeError: unhasable type: 'list'":
allLDR = []
with open('myfile.csv', mode='rb') as f:
reader = csv.reader(f)
#allLDR.extend(reader)
for num, row in enumerate(reader):
if myField in row[0]:
rowNum = row
line_to_override = {rowNum:[nMaisonField, entreeField, indiceField, cadastreField]}
with open('myfile.csv', 'wb') as ofile:
writer = csv.writer(ofile, quoting=csv.QUOTE_NONE, delimiter=',')
#for line, row in enumerate(allLDR):
for line, row in enumerate(reader):
data = line_to_override.get(line, row)
writer.writerow(data)
The line allDR.extend(reader) consumes all of the input lines from the csv.reader object. Therefore, the for loop never runs, and rowNum=row is never executed, and {rowNum:blah} generates an exception.
Try commenting out the allDR.extend(reader) line.
As a debugging aid, try adding print statements inside the for loop and inside the conditional.
Here is a program which does what I think you want your program to do: it reads in myfile.csv, modifies rows conditionally based on the content of the first cell, and writes the file back out.
import csv
with open('myfile.csv', mode='rb') as ifile:
allDR = list(csv.reader(ifile))
for row in allDR:
if 'fowl' in row[0]:
row[:] = ['duck', 'duck', 'goose']
with open('myfile.csv', 'wb') as ofile:
csv.writer(ofile).writerows(allDR)