Reading and Writing CSV Files in Python

Where Do CSV Files Come From?

CSV files are normally created by programs that handle large amounts of data. They are a convenient way to export data from spreadsheets and databases as well as import or use it in other programs. For example, you might export the results of a data mining program to a CSV file and then import that into a spreadsheet to analyze the data, generate graphs for a presentation, or prepare a report for publication.

CSV files are very easy to work with programmatically. Any language that supports text file input and string manipulation (like Python) can work with CSV files directly.

Reading CSV Files With csv

Reading from a CSV file is done using the reader object. The CSV file is opened as a text file with Python’s built-in open() function, which returns a file object. This is then passed to the reader, which does the heavy lifting.

Here’s the employee_birthday.csv file:

name,department,birthday month
John Smith,Accounting,November
Erica Meyers,IT,March

Here’s code to read it:

import csv

with open('employee_birthday.csv') as csv_file:
    csv_reader = csv.reader(csv_file, delimiter=',')
    line_count = 0
    for row in csv_reader:
        if line_count == 0:
            print(f'Column names are {", ".join(row)}')
            line_count += 1
        else:
            print(f'\t{row[0]} works in the {row[1]} department, and was born in {row[2]}.')
            line_count += 1
    print(f'Processed {line_count} lines.')

This results in the following output:

Column names are name, department, birthday month
    John Smith works in the Accounting department, and was born in November.
    Erica Meyers works in the IT department, and was born in March.
Processed 3 lines.

Each row returned by the reader is a list of String elements containing the data found by removing the delimiters. The first row returned contains the column names, which is handled in a special way.

Reading CSV Files Into a Dictionary With csv

Rather than deal with a list of individual String elements, you can read CSV data directly into a dictionary (technically, an Ordered Dictionary) as well.

Again, our input file, employee_birthday.csv is as follows:

name,department,birthday month
John Smith,Accounting,November
Erica Meyers,IT,March

Here’s the code to read it in as a dictionary this time:

import csv

with open('employee_birthday.csv', mode='r') as csv_file:
    csv_reader = csv.DictReader(csv_file)
    line_count = 0
    for row in csv_reader:
        if line_count == 0:
            print(f'Column names are {", ".join(row)}')
            line_count += 1
        print(f'\t{row["name"]} works in the {row["department"]} department, and was born in {row["birthday month"]}.')
        line_count += 1
    print(f'Processed {line_count} lines.')

This results in the same output as before:

Column names are name, department, birthday month
    John Smith works in the Accounting department, and was born in November.
    Erica Meyers works in the IT department, and was born in March.
Processed 3 lines.

Where did the dictionary keys come from? The first line of the CSV file is assumed to contain the keys to use to build the dictionary. If you don’t have these in your CSV file, you should specify your own keys by setting the fieldnames optional parameter to a list containing them.

Optional Python CSV reader Parameters

The reader object can handle different styles of CSV files by specifying additional parameters, some of which are shown below:

• delimiter specifies the character used to separate each field. The default is the comma (',').

• quotechar specifies the character used to surround fields that contain the delimiter character. The default is a double quote (' " ').

• escapechar specifies the character used to escape the delimiter character, in case quotes aren’t used. The default is no escape character.

These parameters deserve some more explanation. Suppose you’re working with the following employee_addresses.csv file:

name,address,date joined
john smith,1132 Anywhere Lane Hoboken NJ, 07030,Jan 4
erica meyers,1234 Smith Lane Hoboken NJ, 07030,March 2

This CSV file contains three fields: name, address, and date joined, which are delimited by commas. The problem is that the data for the address field also contains a comma to signify the zip code.

There are three different ways to handle this situation:

• Use a different delimiter
  That way, the comma can safely be used in the data itself. You use the delimiter optional parameter to specify the new delimiter.

• Wrap the data in quotes
  The special nature of your chosen delimiter is ignored in quoted strings. Therefore, you can specify the character used for quoting with the quotechar optional parameter. As long as that character also doesn’t appear in the data, you’re fine.

• Escape the delimiter characters in the data
  Escape characters work just as they do in format strings, nullifying the interpretation of the character being escaped (in this case, the delimiter). If an escape character is used, it must be specified using the escapechar optional parameter.

Writing CSV Files With csv

You can also write to a CSV file using a writer object and the .write_row() method:

import csv

with open('employee_file.csv', mode='w') as employee_file:
    employee_writer = csv.writer(employee_file, delimiter=',', quotechar='"', quoting=csv.QUOTE_MINIMAL)

    employee_writer.writerow(['John Smith', 'Accounting', 'November'])
    employee_writer.writerow(['Erica Meyers', 'IT', 'March'])

The quotechar optional parameter tells the writer which character to use to quote fields when writing. Whether quoting is used or not, however, is determined by the quoting optional parameter:

• If quoting is set to csv.QUOTE_MINIMAL, then .writerow() will quote fields only if they contain the delimiter or the quotechar. This is the default case.
• If quoting is set to csv.QUOTE_ALL, then .writerow() will quote all fields.
• If quoting is set to csv.QUOTE_NONNUMERIC, then .writerow() will quote all fields containing text data and convert all numeric fields to the float data type.
• If quoting is set to csv.QUOTE_NONE, then .writerow() will escape delimiters instead of quoting them. In this case, you also must provide a value for the escapechar optional parameter.

Reading the file back in plain text shows that the file is created as follows:

John Smith,Accounting,November
Erica Meyers,IT,March

Writing CSV File From a Dictionary With csv

Since you can read our data into a dictionary, it’s only fair that you should be able to write it out from a dictionary as well:

import csv

with open('employee_file2.csv', mode='w') as csv_file:
    fieldnames = ['emp_name', 'dept', 'birth_month']
    writer = csv.DictWriter(csv_file, fieldnames=fieldnames)

    writer.writeheader()
    writer.writerow({'emp_name': 'John Smith', 'dept': 'Accounting', 'birth_month': 'November'})
    writer.writerow({'emp_name': 'Erica Meyers', 'dept': 'IT', 'birth_month': 'March'})

Unlike DictReader, the fieldnames parameter is required when writing a dictionary. This makes sense, when you think about it: without a list of fieldnames, the DictWriter can’t know which keys to use to retrieve values from your dictionaries. It also uses the keys in fieldnames to write out the first row as column names.

The code above generates the following output file:

emp_name,dept,birth_month
John Smith,Accounting,November
Erica Meyers,IT,March

Reading CSV Files With pandas

To show some of the power of pandas CSV capabilities, I’ve created a slightly more complicated file to read, called hrdata.csv. It contains data on company employees:

Name,Hire Date,Salary,Sick Days remaining
Graham Chapman,03/15/14,50000.00,10
John Cleese,06/01/15,65000.00,8
Eric Idle,05/12/14,45000.00,10
Terry Jones,11/01/13,70000.00,3
Terry Gilliam,08/12/14,48000.00,7
Michael Palin,05/23/13,66000.00,8

Reading the CSV into a pandas DataFrame is quick and straightforward:

import pandas
df = pandas.read_csv('hrdata.csv')
print(df)

That’s it: three lines of code, and only one of them is doing the actual work. pandas.read_csv() opens, analyzes, and reads the CSV file provided, and stores the data in a DataFrame. Printing the DataFrame results in the following output:

             Name Hire Date   Salary  Sick Days remaining
0  Graham Chapman  03/15/14  50000.0                   10
1     John Cleese  06/01/15  65000.0                    8
2       Eric Idle  05/12/14  45000.0                   10
3     Terry Jones  11/01/13  70000.0                    3
4   Terry Gilliam  08/12/14  48000.0                    7
5   Michael Palin  05/23/13  66000.0                    8

Here are a few points worth noting:

• First, pandas recognized that the first line of the CSV contained column names, and used them automatically. I call this Goodness.
• However, pandas is also using zero-based integer indices in the DataFrame. That’s because we didn’t tell it what our index should be.

• Further, if you look at the data types of our columns , you’ll see pandas has properly converted the Salary and Sick Days remaining columns to numbers, but the Hire Date column is still a String. This is easily confirmed in interactive mode:

  Python

  >>> print(type(df['Hire Date'][0]))
  <class 'str'>
  

Let’s tackle these issues one at a time. To use a different column as the DataFrame index, add the index_col optional parameter:

import pandas
df = pandas.read_csv('hrdata.csv', index_col='Name')
print(df)

Now the Name field is our DataFrame index:

               Hire Date   Salary  Sick Days remaining
Name                                                  
Graham Chapman  03/15/14  50000.0                   10
John Cleese     06/01/15  65000.0                    8
Eric Idle       05/12/14  45000.0                   10
Terry Jones     11/01/13  70000.0                    3
Terry Gilliam   08/12/14  48000.0                    7
Michael Palin   05/23/13  66000.0                    8

Next, let’s fix the data type of the Hire Date field. You can force pandas to read data as a date with the parse_dates optional parameter, which is defined as a list of column names to treat as dates:

import pandas
df = pandas.read_csv('hrdata.csv', index_col='Name', parse_dates=['Hire Date'])
print(df)

Notice the difference in the output:

                Hire Date   Salary  Sick Days remaining
Name                                                   
Graham Chapman 2014-03-15  50000.0                   10
John Cleese    2015-06-01  65000.0                    8
Eric Idle      2014-05-12  45000.0                   10
Terry Jones    2013-11-01  70000.0                    3
Terry Gilliam  2014-08-12  48000.0                    7
Michael Palin  2013-05-23  66000.0                    8

The date is now formatted properly, which is easily confirmed in interactive mode:

>>> print(type(df['Hire Date'][0]))
<class 'pandas._libs.tslibs.timestamps.Timestamp'>

If your CSV files doesn’t have column names in the first line, you can use the names optional parameter to provide a list of column names. You can also use this if you want to override the column names provided in the first line. In this case, you must also tell pandas.read_csv() to ignore existing column names using the header=0 optional parameter:

import pandas
df = pandas.read_csv('hrdata.csv', 
            index_col='Employee', 
            parse_dates=['Hired'], 
            header=0, 
            names=['Employee', 'Hired','Salary', 'Sick Days'])
print(df)

Notice that, since the column names changed, the columns specified in the index_col and parse_dates optional parameters must also be changed. This now results in the following output:

                    Hired   Salary  Sick Days
Employee                                     
Graham Chapman 2014-03-15  50000.0         10
John Cleese    2015-06-01  65000.0          8
Eric Idle      2014-05-12  45000.0         10
Terry Jones    2013-11-01  70000.0          3
Terry Gilliam  2014-08-12  48000.0          7
Michael Palin  2013-05-23  66000.0          8

Writing CSV Files With pandas

Of course, if you can’t get your data out of pandas again, it doesn’t do you much good. Writing a DataFrame to a CSV file is just as easy as reading one in. Let’s write the data with the new column names to a new CSV file:

import pandas
df = pandas.read_csv('hrdata.csv', 
            index_col='Employee', 
            parse_dates=['Hired'],
            header=0, 
            names=['Employee', 'Hired', 'Salary', 'Sick Days'])
df.to_csv('hrdata_modified.csv')

The only difference between this code and the reading code above is that the print(df) call was replaced with df.to_csv(), providing the file name. The new CSV file looks like this:

Employee,Hired,Salary,Sick Days
Graham Chapman,2014-03-15,50000.0,10
John Cleese,2015-06-01,65000.0,8
Eric Idle,2014-05-12,45000.0,10
Terry Jones,2013-11-01,70000.0,3
Terry Gilliam,2014-08-12,48000.0,7
Michael Palin,2013-05-23,66000.0,8