I am asking Python to print the minimum number from a column of CSV data, but the top row is the column number, and I don't want Python to take the top row into account. How can I make sure Python ignores the first line?
This is the code so far:
import csv
with open('all16.csv', 'rb') as inf:
incsv = csv.reader(inf)
column = 1
datatype = float
data = (datatype(column) for row in incsv)
least_value = min(data)
print least_value
Could you also explain what you are doing, not just give the code? I am very very new to Python and would like to make sure I understand everything.
You could use an instance of the csv module's Sniffer class to deduce the format of a CSV file and detect whether a header row is present along with the built-in next() function to skip over the first row only when necessary:
import csv
with open('all16.csv', 'r', newline='') as file:
has_header = csv.Sniffer().has_header(file.read(1024))
file.seek(0) # Rewind.
reader = csv.reader(file)
if has_header:
next(reader) # Skip header row.
column = 1
datatype = float
data = (datatype(row[column]) for row in reader)
least_value = min(data)
print(least_value)
Since datatype and column are hardcoded in your example, it would be slightly faster to process the row like this:
data = (float(row[1]) for row in reader)
Note: the code above is for Python 3.x. For Python 2.x use the following line to open the file instead of what is shown:
with open('all16.csv', 'rb') as file:
To skip the first line just call:
next(inf)
Files in Python are iterators over lines.
Borrowed from python cookbook,
A more concise template code might look like this:
import csv
with open('stocks.csv') as f:
f_csv = csv.reader(f)
headers = next(f_csv)
for row in f_csv:
# Process row ...
In a similar use case I had to skip annoying lines before the line with my actual column names. This solution worked nicely. Read the file first, then pass the list to csv.DictReader.
with open('all16.csv') as tmp:
# Skip first line (if any)
next(tmp, None)
# {line_num: row}
data = dict(enumerate(csv.DictReader(tmp)))
You would normally use next(incsv) which advances the iterator one row, so you skip the header. The other (say you wanted to skip 30 rows) would be:
from itertools import islice
for row in islice(incsv, 30, None):
# process
use csv.DictReader instead of csv.Reader.
If the fieldnames parameter is omitted, the values in the first row of the csvfile will be used as field names. you would then be able to access field values using row["1"] etc
Python 2.x
csvreader.next()
Return the next row of the reader’s iterable object as a list, parsed
according to the current dialect.
csv_data = csv.reader(open('sample.csv'))
csv_data.next() # skip first row
for row in csv_data:
print(row) # should print second row
Python 3.x
csvreader.__next__()
Return the next row of the reader’s iterable object as a list (if the
object was returned from reader()) or a dict (if it is a DictReader
instance), parsed according to the current dialect. Usually you should
call this as next(reader).
csv_data = csv.reader(open('sample.csv'))
csv_data.__next__() # skip first row
for row in csv_data:
print(row) # should print second row
The documentation for the Python 3 CSV module provides this example:
with open('example.csv', newline='') as csvfile:
dialect = csv.Sniffer().sniff(csvfile.read(1024))
csvfile.seek(0)
reader = csv.reader(csvfile, dialect)
# ... process CSV file contents here ...
The Sniffer will try to auto-detect many things about the CSV file. You need to explicitly call its has_header() method to determine whether the file has a header line. If it does, then skip the first row when iterating the CSV rows. You can do it like this:
if sniffer.has_header():
for header_row in reader:
break
for data_row in reader:
# do something with the row
this might be a very old question but with pandas we have a very easy solution
import pandas as pd
data=pd.read_csv('all16.csv',skiprows=1)
data['column'].min()
with skiprows=1 we can skip the first row then we can find the least value using data['column'].min()
The new 'pandas' package might be more relevant than 'csv'. The code below will read a CSV file, by default interpreting the first line as the column header and find the minimum across columns.
import pandas as pd
data = pd.read_csv('all16.csv')
data.min()
Because this is related to something I was doing, I'll share here.
What if we're not sure if there's a header and you also don't feel like importing sniffer and other things?
If your task is basic, such as printing or appending to a list or array, you could just use an if statement:
# Let's say there's 4 columns
with open('file.csv') as csvfile:
csvreader = csv.reader(csvfile)
# read first line
first_line = next(csvreader)
# My headers were just text. You can use any suitable conditional here
if len(first_line) == 4:
array.append(first_line)
# Now we'll just iterate over everything else as usual:
for row in csvreader:
array.append(row)
Well, my mini wrapper library would do the job as well.
>>> import pyexcel as pe
>>> data = pe.load('all16.csv', name_columns_by_row=0)
>>> min(data.column[1])
Meanwhile, if you know what header column index one is, for example "Column 1", you can do this instead:
>>> min(data.column["Column 1"])
For me the easiest way to go is to use range.
import csv
with open('files/filename.csv') as I:
reader = csv.reader(I)
fulllist = list(reader)
# Starting with data skipping header
for item in range(1, len(fulllist)):
# Print each row using "item" as the index value
print (fulllist[item])
I would convert csvreader to list, then pop the first element
import csv
with open(fileName, 'r') as csvfile:
csvreader = csv.reader(csvfile)
data = list(csvreader) # Convert to list
data.pop(0) # Removes the first row
for row in data:
print(row)
I would use tail to get rid of the unwanted first line:
tail -n +2 $INFIL | whatever_script.py
just add [1:]
example below:
data = pd.read_csv("/Users/xyz/Desktop/xyxData/xyz.csv", sep=',', header=None)**[1:]**
that works for me in iPython
Python 3.X
Handles UTF8 BOM + HEADER
It was quite frustrating that the csv module could not easily get the header, there is also a bug with the UTF-8 BOM (first char in file).
This works for me using only the csv module:
import csv
def read_csv(self, csv_path, delimiter):
with open(csv_path, newline='', encoding='utf-8') as f:
# https://bugs.python.org/issue7185
# Remove UTF8 BOM.
txt = f.read()[1:]
# Remove header line.
header = txt.splitlines()[:1]
lines = txt.splitlines()[1:]
# Convert to list.
csv_rows = list(csv.reader(lines, delimiter=delimiter))
for row in csv_rows:
value = row[INDEX_HERE]
Simple Solution is to use csv.DictReader()
import csv
def read_csv(file): with open(file, 'r') as file:
reader = csv.DictReader(file)
for row in reader:
print(row["column_name"]) # Replace the name of column header.
Related
I tried sever times to use strip() but I can't get it to work.
I removed that piece from this snip but every time I tried it I had
an error or it did nothing. The sort is fine I just want to strip the newline before writing to the new file?
import sys, csv, operator
data = csv.reader(open('tickets.csv'),delimiter=',')
sortedlist = sorted(data, key=operator.itemgetter(6))
# 0 specifies according to first column we want to sort
#now write the sort result into new CSV file
with open("newfiles.csv", "w") as f:
#writablefile = csv.writer(f)
fileWriter = csv.writer(f, delimiter=',')
for row in sortedlist:
#print(row)
lst = (row)
fileWriter.writerow(lst)
You need to add newline='' to your open() when writing a CSV file. This is explained in the documentation. Without it, your file can end up having a blank line per row.
import sys, csv, operator
data = csv.reader(open('tickets.csv'),delimiter=',')
header = next(data)
sortedlist = sorted(data, key=operator.itemgetter(6))
# 0 specifies according to first column we want to sort
#now write the sort result into a new CSV file
with open("newfiles.csv", "w", newline="") as f:
fileWriter = csv.writer(f)
fileWriter.writerow(header) # keep the header at the top
fileWriter.writerows(sortedlist)
Also you need to first read in the header row before loading everything for sorting. This avoids it being sorted. It can then be output separately when writing your sorted output CSV.
If your tickets.csv file contains blank lines, you would need to also remove these. For example:
for row in sortedList:
if row:
fileWriter.writerow(row)
I am a beginner of Python and would like to have your opinion..
I wrote this code that reads the only column in a file on my pc and puts it in a list.
I have difficulties understanding how I could modify the same code with a file that has multiple columns and select only the column of my interest.
Can you help me?
list = []
with open(r'C:\Users\Desktop\mydoc.csv') as file:
for line in file:
item = int(line)
list.append(item)
results = []
for i in range(0,1086):
a = list[i-1]
b = list[i]
c = list[i+1]
results.append(b)
print(results)
You can use pandas.read_csv() method very simply like this:
import pandas as pd
my_data_frame = pd.read_csv('path/to/your/data')
results = my_data_frame['name_of_your_wanted_column'].values.tolist()
A useful module for the kind of work you are doing is the imaginatively named csv module.
Many csv files have a "header" at the top, this by convention is a useful way of labeling the columns of your file. Assuming you can insert a line at the top of your csv file with comma delimited fieldnames, then you could replace your program with something like:
import csv
with open(r'C:\Users\Desktop\mydoc.csv') as myfile:
csv_reader = csv.DictReader(myfile)
for row in csv_reader:
print ( row['column_name_of_interest'])
The above will print to the terminal all the values that match your specific 'column_name_of_interest' after you edit it to match your particular file.
It's normal to work with lots of columns at once, so that dictionary method of packing a whole row into a single object, addressable by column-name can be very convenient later on.
To a pure python implementation, you should use the package csv.
data.csv
Project1,folder1/file1,data
Project1,folder1/file2,data
Project1,folder1/file3,data
Project1,folder1/file4,data
Project1,folder2/file11,data
Project1,folder2/file42a,data
Project1,folder2/file42b,data
Project1,folder2/file42c,data
Project1,folder2/file42d,data
Project1,folder3/filec,data
Project1,folder3/fileb,data
Project1,folder3/filea,data
Your python program should read it by line
import csv
a = []
with open('data.csv') as csv_file:
reader = csv.reader(csv_file, delimiter=',')
for row in reader:
print(row)
# ['Project1', 'folder1/file1', 'data']
If you print the row element you will see it is a list like that
['Project1', 'folder1/file1', 'data']
If I would like to put in my list all elements in column 1, I need to put that element in my list, doing:
a.append(row[1])
Now in list a I will have a list like:
['folder1/file1', 'folder1/file2', 'folder1/file3', 'folder1/file4', 'folder2/file11', 'folder2/file42a', 'folder2/file42b', 'folder2/file42c', 'folder2/file42d', 'folder3/filec', 'folder3/fileb', 'folder3/filea']
Here is the complete code:
import csv
a = []
with open('data.csv') as csv_file:
reader = csv.reader(csv_file, delimiter=',')
for row in reader:
a.append(row[1])
I'm trying to access a csv file of currency pairs using csv.reader. The first column shows dates, the first row shows the currency pair eg.USD/CAD. I can read in the file but cannot access the currency pairs data to perform simple calculations.
I've tried using next(x) to skip header row (currency pairs). If i do this, i get a Typeerror: csv reader is not subscriptable.
path = x
file = open(path)
dataset = csv.reader(file, delimiter = '\t',)
header = next(dataset)
header
Output shows the header row which is
['Date,USD,Index,CNY,JPY,EUR,KRW,GBP,SGD,INR,THB,NZD,TWD,MYR,IDR,VND,AED,PGK,HKD,CAD,CHF,SEK,SDR']
I expect to be able to access the underlying currency pairs but i'm getting the type error as noted above. Is there a simple way to access the currency pairs, for example I want to use USD.describe() to get simple statistics on the USD currency pair.
How can i move from this stage to accessing the data underlying the header row?
try this example
import csv
with open('file.csv') as csv_file:
csv_reader = csv.Reader(csv_file, delimiter='\t')
line_count = 0
for row in csv_reader:
print(f'\t{row[0]} {row[1]} {row[3]}')
It's apparent from the output of your header row that the columns are comma-delimited rather than tab-delimited, so instead of passing delimiter = '\t' to csv.reader, you should let it use the default delimiter ',' instead:
dataset = csv.reader(file)
If you need to elaborate some statistics pandas is your friend. No need to use the csv module, use pandas.read_csv.
import pandas
filename = 'path/of/file.csv'
dataset = pandas.read_csv(filename, sep = '\t') #or whatever the separator is
pandas.read_csv uses the first line as the header automatically.
To see statistics, simply do:
dataset.describe()
Or for a single column:
dataset['column_name'].describe()
Are you sure that your delimiter is '\t'? In first row your delimiter is ','... Anyway you can skip first row by doing file.readline() before using it by csv.reader:
import csv
example = """Date,USD,Index,CNY,JPY,EUR,KRW,GBP,SGD,INR,THB,NZD,TWD,MYR,IDR,VND,AED,PGK,HKD,CAD,CHF,SEK,SDR
1-2-3\tabc\t1.1\t1.2
4-5-6\txyz\t2.1\t2.2
"""
with open('demo.csv', 'w') as f:
f.write(example)
with open('demo.csv') as f:
f.readline()
reader = csv.reader(f, delimiter='\t')
for row in reader:
print(row)
# ['1-2-3', 'abc', '1.1', '1.2']
# ['4-5-6', 'xyz', '2.1', '2.2']
I think that you need something else... Can you add to your question:
example of first 3 lines in your csv
Example of what you'd like to access:
is using row[0], row[1] enough for you?
or do you want "named" access like row['Date'], row['USD'],
or you want something more complex like data_by_date['2019-05-01']['USD']
I'm modifying a CSV file with two fieldnames and I would like to add a string to the end of each row under an additional fieldname. I've figured out how to add the fieldname, but not how to add things to it. It seems like it should be simple, but I'm stumped.
import csv
with open('test2l.csv', 'r') as inny:
reader = csv.DictReader(inny)
with open('outfile.csv', 'w') as outty:
fieldnames = ["Genus", "Species", "Source"]
writer = csv.DictWriter(outty, fieldnames = fieldnames)
writer.writeheader()
for record in reader:
g = record['Genus']
s = record['Species']
Everything I tried has just added the string to the existing string in 'Species' and I haven't been able to create a record for 'Source', I assume because it's empty.
Thanks!
If you haven't already, check out the documentation for csv.Reader, csv.DictReader, and csv.DictWriter.
The documentation indicates that reader objects operate on an on object using the iterator protocol. Iterating once (for row in reader:, for example) to add a "Source" exhausts the underlying iterator. Attempting to then use the same reader with a writer later would not work.
To work around this, you could create a list:
rows = list(csv.DictReader(inny))
While this exhausts the iterator, you now have a list to work with. However, this might not be ideal if the list is very long.
Another solution would be to add the Source and write the row during the same iteration:
for row in reader:
row['Source'] = 'same string every time'
writer.writerow(row)
Simply write:
for record in reader:
record["Source"] = "whateversource"
And do so before the step where you are writing to a file.`
I'm trying to create a list out of the first row - the column headers - of a csv file using python. I put together this little script, but it prints two different lists. The first item printed is the first row of the csv and the second thing printed in the second row.
What am I doing wrong?
import csv
import sys
with open('agentsFullOutput.csv') as csvFile:
reader = csv.reader(csvFile)
print csvFile.next()
field_names_list = []
field_names_list = csvFile.next()
print field_names_list
Every time you call .next() it'll move on to the next row in the file. So you'll only get the headers of the CSV file in the first .next() call:
import csv
with open('agentsFullOutput.csv') as csvFile:
reader = csv.reader(csvFile)
field_names_list = reader.next()
Any subsequent .next() call will read the next row in the file so that would be a row of data.
Each time you call next() the next line from the file is yielded until the end of the file is reached.
In your code example, since you call next() twice, and in the second call you assign it to field_name_list, it assigns the 2nd row, not the first.
The following will assign the first row to the variable field_names_list.
with open('agentsFullOutput.csv') as csvFile:
reader = csv.reader(csvFile)
field_names_list = next(reader)
Using next(reader) instead of reader.next() means that the code is portable to Python 3. On the 3+ series, the iterator's next() method has been renamed to __next__() for consistency.
You can use the Pandas library to read the first few lines from the huge dataset.
import pandas as pd
data = pd.read_csv("names.csv", nrows=1)
You can mention the number of lines to be read in the nrows parameter.
with open(__csvfile) as csvFile:
reader = csv.DictReader(csvFile)