I am trying to read and write on the same CSV file:
file1 = open(file.csv, 'rb')
file2 = open(file.csv, 'wb')
reader = csv.reader(file1)
writer = csv.writer(file2)
for row in reader:
if row[2] == 'Test':
writer.writerow( row[0], row[1], 'Somevalue')
My csv files are:
val1,2323,Notest
val2, 2323,Test
So basically if my row[2] value is Test I want to replace it with Some new value.
The above code gives me empty CSV files.
You should use different output file name. Even if you want the name to be the same, you should use some temporary name and finally rename file.
When you open file in 'w' (or 'wb') mode this file is "cleared" -- whole file content disappears. Python documentation for open() says:
... 'w' for only writing (an existing file with the same name will be erased), ...
So your file is erased before csv functions start parsing it.
You can't open a file in both read and write modes at once.
Your code could be modified as follows:-
# Do the reading
file1 = open(file.csv, 'rb')
reader = csv.reader(file1)
new_rows_list = []
for row in reader:
if row[2] == 'Test':
new_row = [row[0], row[1], 'Somevalue']
new_rows_list.append(new_row)
file1.close() # <---IMPORTANT
# Do the writing
file2 = open(file.csv, 'wb')
writer = csv.writer(file2)
writer.writerows(new_rows_list)
file2.close()
As Jason points out, if your CSV is too big for your memory, then you'll need to write to a different filename and then rename it. This will likely be a bit slower.
If your csv file is not big enough(to explode the memory), read it all into memory and close the file before open it in write mode.
Or you should consider writing to a new file rather than the same one.
It is not possible to open the same file in two different modes in python.You have to release one of the file pointers with file_name.close() before opening the file in another mode!
Related
I'm trying to read sentences in a csv file, convert them to lowercase and save in other csv file.
import csv
import pprint
with open('dataset_elec_4000.csv') as f:
with open('output.csv', 'w') as ff:
data = f.read()
data = data.lower
writer = csv.writer(ff)
writer.writerow(data)
but I got error "_csv.Error: sequence expected". What should I do?
*I'm a beginner. Please be nice to me:)
You need to read over your input CSV row-by-row, and for each row, transform it, then write it out:
import csv
with open('output.csv', 'w', newline='') as f_out:
writer = csv.writer(f_out)
with open('dataset_elec_4000.csv', newline='') as f_in:
reader = csv.reader(f_in)
# comment these two lines if no input header
header = next(reader)
writer.writerow(header)
for row in reader:
# row is sequence/list of cells, so...
# select the cell with your sentence, I'm presuming it's the first cell (row[0])
data = row[0]
data = data.lower()
# need to put data back into a "row"
out_row = [data]
writer.writerow(out_row)
Python contains a module called csv for the handling of CSV files. The reader class from the module is used for reading data from a CSV file. At first, the CSV file is opened using the open() method in ‘r’ mode(specifies read mode while opening a file) which returns the file object then it is read by using the reader() method of CSV module that returns the reader object that iterates throughout the lines in the specified CSV document.
import csv
# opening the CSV file
with open('Giants.csv', mode ='r')as file:
# reading the CSV file
csvFile = csv.reader(file)
# displaying the contents of the CSV file
for lines in csvFile:
print(lines)
I have a csv file and I want to transfer the raw data without the headers to a new csv file and have the rows and columns the same as the original.
IRIS_data = "IRIS_data.csv"
with open(IRIS_data, 'wb') as data:
wr = csv.writer(data, quoting=csv.QUOTE_ALL)
with open(IRIS) as f:
next(f)
for line in f:
wr.writerow(line)
The code above is my most recent attempt, when I try run it I get the following error:
a bytes-like object is required, not 'str'
It's because you opened the input file with with open(IRIS_data, 'wb'), which opens it in binary mode, and the output file with just with open(IRIS) which opens it in text mode.
In Python 3, you should open both files in text mode and specify newline='' option)—see the examples in the csv module's documentation)
To fix it, change them as follows:
with open(IRIS_data, 'w', newline='') as data:
and
with open(IRIS, newline='') as f:
However there are other issues with you code. Here's how to use those statements to get what I think you want:
import csv
IRIS = "IRIS.csv"
IRIS_data = "IRIS_data.csv"
with open(IRIS, 'r', newline='') as f, open(IRIS_data, 'w', newline='') as data:
next(f) # Skip over header in input file.
writer = csv.writer(data, quoting=csv.QUOTE_ALL)
writer.writerows(line.split() for line in f)
Contents of IRIS_data.csv file after running the script with your sample input data:
"6.4","2.8","5.6","2.2","2"
"5","2.3","3.3","1","1"
"4.9","2.5","4.5","1.7","2"
"4.9","3.1","1.5","0.1","0"
"5.7","3.8","1.7","0.3","0"
"4.4","3.2","1.3","0.2","0"
"5.4","3.4","1.5","0.4","0"
"6.9","3.1","5.1","2.3","2"
"6.7","3.1","4.4","1.4","1"
"5.1","3.7","1.5","0.4","0"
You have to encode the line you are writing like this:
wr.writerow( line.encode(”utf8”))
Also open your file using open(..., ‘wb’). This will open the file in binary mode. So you are certain the file is actually open in binary mode. Indeed it is better to now explicitly the encoding than assuming it. Enforcing encoding for both reading and writing will save you lots of trouble.
I am trying to create .csv file.
For some reason it skips line before printing entry.
Here is the output
But here is what I need
Below is code. Apparently if line != "": doesn't work
import csv
#-----------------------------------
def csv_writer(data,path):
"""
Write data to a CSV file path
"""
with open(path, "w") as csv_file:
writer = csv.writer(csv_file, delimiter=',')
for line in data:
if line != "":
writer.writerow(line)
#-----------------------------------
if __name__ == "__main__":
data = ["first_name,last_name,city".split(","),
"Tyrese,Hirthe,Strackeport".split(","),
"Jules,Dicki,Lake Nickolasville".split(","),
"Dedric,Medhurst,Stiedemannberg".split(",")
]
path = "output.csv"
csv_writer(data,path)
Some python versions (on windows) have an issue with that with open(path, "w") as csv_file:. A spurious carriage return char is inserted, creating a blank line after each line.
You have to add newline="" as stated in the documentation. Python 3:
with open(path, "w",newline="") as csv_file:
writer = csv.writer(csv_file, delimiter=',')
As for python 2:
with open(path, "wb") as csv_file:
writer = csv.writer(csv_file, delimiter=',')
see also:
portable way to write csv file in python 2 or python 3
csv writer expected byte like and space between rows
(note that latest Python versions on Windows don't need this anymore, but the documentation continues to state it)
When you open the file you need to pass the keyword argument newline with a blank string. This will prevent the newlines being added between rows. Your function should be:
def csv_writer(data,path):
"""
Write data to a CSV file path
"""
with open(path, "w", newline = '') as csv_file:
writer = csv.writer(csv_file, delimiter=',')
for line in data:
if line:
writer.writerow(line)
Note that this is only an issue on Windows.
I'm trying to read the rows of a CSV file and print each row, but the code I've been using isn't opening the file or running the FOR loop. Any ideas?
import csv
domainFile = 'magtest.csv'
f = open(domainFile, 'ab+')
try:
reader = csv.reader(f)
print "file opened"
for row in reader:
print "Read domain: %s" %row
finally:
f.close()
ab+ mode opens a file for both appending and binary format, so therefore you cannot read in the contents if it is open to be appended to, you want r to read it:
f = open(domainFile, 'r')
For more information about all of the different file modes, please refer to this documentation.
I am trying to write to a .tsv file using python's CSV module, this is my code so far
file_name = "test.tsv"
TEMPLATE = "template.tsv"
fil = open(file_name, "w")
# Added suggested change
template = csv.DictReader(open(TEMPLATE, 'r'), delimiter='\t')
new_file = csv.DictWriter(fil, fieldnames=template.fieldnames, delimiter='\t')
new_file.writeheader()
basically TEMPLATE is a file that will contain the headers for the file, so i read the headers using DictReader and pass the fieldnames to DictWriter, as far as i know the code is fine, the file test.tsv is being created but for some reason the headers are not being written.
Any help as to why this is happening is appreciated, thanks.
DictReader's first argument should be a file object (create with open()), cf. http://docs.python.org/py3k/library/csv.html#csv.DictReader
You forgot open() for the TEMPLATE file.
import csv
file_name = "test.tsv"
TEMPLATE = "template.tsv"
fil = open(file_name, "w")
# you forgot this line, which will open the file
template_file = open(TEMPLATE, 'r')
template = csv.DictReader(template_file, delimiter='\t')
new_file = csv.DictWriter(fil, fieldnames=template.fieldnames, delimiter='\t')
new_file.writeheader()
Try to give DictReader opened file instead of file name:
csv.DictReader(open(TEMPLATE, 'r'), delimiter='\t')
Same for the writer, but opened for writing.