Csv python-project

File Reading and Writing
COMSATS INSTITUTE OF INFORMATION TECHNOLOGY, ISLAMABAD
1

CSV
 The spreadsheet is a very popular, and powerful, application for
manipulating data
 Its popularity means there are many companies that provide their own
version of the spreadsheet
 It would be nice if those different versions could share their data
2

CONT…..
 A basic approach to share data is the comma separated value
(CSV) format
it is a text format, accessible to all apps
each line (even if blank) is a row
in each row, each value is separated from the others by a
comma (even if it is blank)
cannot capture complex things like formula
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Spread sheet and corresponding
CSV file
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CSV format
 As simple as that sounds, even CSV format is not completely
universal ,different apps have small variations
 Python provides a module to deal with these variations called
the csv module
 This module allows you to read spreadsheet info into your
program
 We load the module in the usual way using import:
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CONT…….
 As in our prior knowledge python usually reads a text file as a line at a time.
 each line ends with an “end of line” (EOL) indicator, On Unix and Linux the EOL
indicator is the new line character (‘n’). On Windows it is actually two characters,
one of which is the new line character.
 csv module can handle CSV files correctly regardless of the operating system on
which the files were created.
 special handling of the EOL indicator for CSV files, using a special option when we
open the file: newline=''
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CONT…
 we would open a CSV file for reading like
this:
 we would open a new CSV file for writing
like this:
“newline” is used for switching to next line(row)
while entering data in row.
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CSV.READER
 Return a reader object which will iterate over lines in the
given csv file.
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EXAMPLE PATIENT.CSV
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CSV.READER
 First off, we have to actually import the csv module.
 Then we create a very simple function called csv_reader that
accepts a file object.
 Inside the function, we pass the file object into
the csv_reader function, which returns a reader object.
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WITH CSV.READER
 Read each row in form of list
WITHOUT CSV.READER
 Simply prints
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READING FROM A CSV FILE
 How do we read from a CSV file ?
 We open the file (in text mode) for reading (making sure we give open())
 We create a special type of object to access the CSV file (reader object) d which
we create using the reader() function
 The reader object is an iterable that gives us access to each line of the CSV file as
a list of fields we can use next() directly on it to read the next line of the CSV file,
or we can treat it like a list in a for loop to read all the lines of the file (as lists of the
file’s fields).
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CONT…..
 When we’ve finished reading from the file we delete the reader object and
then close the file
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PRINTING SPECIFIC COLOUMN
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“ ”.join(row) ‘,’.join(row)
join each element in the row together
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QUOTING
 The csv module contains a the following quoting options.
 csv.QUOTE_ALL Quote everything, regardless of type.
 csv.QUOTE_MINIMAL Quote fields with special characters
 csv.QUOTE_NONNUMERIC Quote all fields that are not integers or floats
 csv.QUOTE_NONE Do not quote anything on output
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SLICING
• Use a range to specify a slice (sub-data)
Format: sample[start : end]
Includes the start index but excludes the last index.
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ALLOTTING ROW NO. BY USING
“LINE_NUM”
The print() function call prints the number of
the current row and the contents of the row. To
get the row number, use
the Reader object’s line_num variable, which
contains the number of the current line.
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EXAMPLES USE OF LINE_NUM
 Skipping specific row
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LISTING THE DATA
 Using list() on this Reader object returns a list of lists, which you can
store in a variable like data. Entering data in the shell displays the list of
lists
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LIST COMPREHENSION
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DICT READER
When iterate over a CSV file, each
iteration of the loop produces a
dictionary. They keys are the names of
the columns (from the first row of the file,
which is skipped over), and the values
are the data from the row being read.
22

WRITING A CSV FILE
 How do we write to one?
 We open the file (in text mode) for writing (making sure we give open() the newline=''
option).
 We create a special type of object to write to the CSV file “writer object”, which is
defined in the csv module, and which we create using the writer() function
 The writerow() method, that allows us to write a list of fields to the file. The fields can be
strings or numbers or both writerow() will convert them if necessary
 When using writerow() you do not add a new line character (or other EOL indicator) to
indicate the end of the line, writerow() does it for you as necessary
fw= open('output.csv', 'w', newline='')
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WRITING USING “WRITEROW”
A Writer object lets you write data to a CSV file. To create a Writer object, you use
the csv.writer() function.
The writerow() method for Writer objects takes a list argument. Each value in the
list is placed in its own cell in the output CSV file.
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TAKING RUN TIME INPUT IN FILE
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USE OF DELIMITER AND
LINE_TERMINATOR
• If you want to separate cells with a tab character instead of a comma and you want
the rows to be double-spaced.
• Use delimiter and line terminator.
• Passing delimiter='t' and line terminator='nn' changes the character between
cells to a tab and the character between rows to two newlines.
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Write Dictionary in
csv.format
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