Comma-Separated Values (CSV) is the most common text file format that is used for importing and exporting data to and from spreadsheets and databases. The general format says that each line contains one record (a record is a complete entity), listing each of the fields of the record in a specific order, separating the fields by commas. The first line of the file may or may not consist of names for the fields in the CSV file.
The code below loads and displays the contents of a typical CSV file. Appendix 331 explains how to get this CSV file.
fp = open( "pc_inventory.csv" )
print( fp.read().strip() )
fp.close()
Unfortunately, the CSV format is not standardized, and different
software packages tend to use slightly different implementations of CSV
files. However, over the years the different conventions used by the
major software packages have converged to something of a standard, that
is implemented in the Python csv module. The module supports
“dialects” of CSV formats to handle files from different sources.
Instead of using the csv module, if you have to deal with an excentric
CSV format that the module does not support, you can try to design your
own interpretation of lines of the file using regular expressions. You
can also try to design your own dialect. Neither option is very
appealing.
reader()The csv module contains a reader() function that provides access to
a CSV file. The reader() function gets a file handle as argument, and
returns an iterator that allows you to get the lines from the file, as a
list with each of the fields as an element of the list. You should leave
the file open while accessing it with reader().
from csv import reader
fp = open( "pc_inventory.csv", newline='' )
csvreader = reader( fp )
for line in csvreader:
print( line )
fp.close()
The Python documentation recommends that if you use reader() on a file
(and that is what you usually do), you specify a newline='' argument
as extra argument when opening the file (I did this in the code above).
This is necessary in case some of the text fields in the CSV file
contain newline characters.
reader() takes extra arguments too. Common ones are
delimiter=<character>, which indicates the <character> which is
placed between different fields (default is the comma), and
quotechar=<character>, which indicates which <character> is used to
enclose strings with (default is the double quote).
writer()Writing a CSV file is just a little bit harder than reading one. You
create a file handle to a file that you open for writing ("w" mode),
and use it as an argument when you call the writer() function from the
csv module. The object that is returned from the writer() call has a
method writerow() that you can call with a list of fields, that it
then writes to the output file in CSV format.
The call to writer() can get the same arguments as the call to
reader() can get, including specifying a delimiter and a
quotechar. You can also supply a quoting=<quotemethod> argument,
that supports the following methods of quoting:
csv.QUOTE_ALL, which encloses every field in quotation characters
csv.QUOTE_MINIMAL, which only encloses fields in quotation
characters if it is absolutely necessary (this is the default)
csv.QUOTE_NONNUMERIC, which encloses fields in quotation
characters if they are not integers or floats
csv.QUOTE_NONE, which encloses no fields in quotation characters
Enclosing a string within quotation characters is generally only needed if the string contains exceptional characters, such as newlines or the character that is used as delimiter.
from csv import writer
fp = open( "pc_writetest.csv", "w", newline='' )
csvwriter = writer( fp )
csvwriter.writerow( ["MOVIE", "RATING"] )
csvwriter.writerow( ["Monty Python and the Holy Grail", 8] )
csvwriter.writerow( ["Monty Python's Life of Brian", 8.5] )
csvwriter.writerow( ["Monty Python's Meaning of Life", 7] )
fp.close()
After using the code above to create the file “pc_writetest.csv,” open
it and use reader() to list its contents.