How does the Python CSV Sniffer work

How to read and write CSV files in Python

The CSV format is the most commonly used import and export format for databases and spreadsheets. This tutorial provides a detailed introduction to CSVs and the modules and classes available for reading and writing data to CSV files. It also describes a working example showing how to write and read data to a CSV file in Python.

What is a CSV file?

A comma separated values ​​(CSV) file allows data to be stored in a table structure with a .csv extension. CSV files have been widely used in ecommerce applications because they are considered very easy to process. Some of the areas in which they have been used include:

  • Import and export customer data
  • Import and export of products
  • Export orders
  • Exporting e-commerce analytics reports

Reader and writer modules

The CSV module has several functions and classes for reading and writing CSVs. This includes:

  • csv.reader function
  • csv.writer function
  • csv.Dictwriter class
  • csv.DictReader class


The csv.reader module requires the following parameters:

  • : This is usually an object that supports the iterator protocol and usually returns a string every time method is called.
  • : An optional parameter used to define a parameter set for a particular CSV dialect.
  • : An optional parameter that can be used to overwrite existing formatting parameters.

Here is an example of using the csv.reader module.

Import csv with open ('example.csv', newline = ") as file: reader = csv.reader (file) for line in reader: print (line)

csv.writer module

This module is similar to the csv.reader module and is used to write data to a CSV. There are three parameters required:

  • : These can be any object with a method.
  • : An optional parameter used to define a parameter set for a particular CSV.
  • : An optional parameter that can be used to overwrite existing formatting parameters.

DictReader and DictWriter classes

DictReader and DictWriter are classes available in Python for reading and writing in CSV. Although similar to Reader and Writer functions, these classes use dictionary objects to read and write to CSV files.


It creates an object that maps the information read into a dictionary, the key of which is passed by the parameter. This parameter is optional. However, if it is not specified in the file, the data on the first line becomes the keys of the dictionary.


Import csv with open ('name.csv') as csvfile: reader = csv.DictReader (csvfile) for the line in the reader: print (row ['first_name'], line ['last_name'])


This class is similar to the DictWriter class and does the opposite. In doing so, data is written to a CSV file. The class is defined as

The parameter defines the order of the keys, which specify the order in which values ​​in the dictionary are written to the CSV file. In contrast to the DictReader, this key is not optional and must be defined in order to avoid errors when writing to a CSV.

Dialects and formatting

A dialect is an auxiliary class that is used to define the parameters for a specific object or example. Dialects and formatting parameters must be declared when executing a read or write function.

There are several attributes that a dialect supports:

  • delimiter: A string used to separate fields. The default is.
  • double quotes: Controls how instances of quotechar appear within a field should be placed in quotation marks. Can be true or false.
  • escapechar: A string used by the writer to bypass the string delimiter if quote is set to.
  • Line terminator: A string used to end lines. The default is.
  • quotechar: A string used to specify fields with special characters. The default is.
  • Skipititialspace: When set to, every white space immediately after delimiter is ignored.
  • strict: If set to, an exception is thrown if the CSV entry is incorrect.
  • quote: Controls when quotations are to be generated when reading or writing to a CSV.

Read a CSV file

Let's see how to read a CSV file using the help modules discussed above.

Create your CSV file and save it as example.csv. Make sure it's the extension and fill in some data. Here we have our CSV file that contains the names of the students and their grades.

Below is the code to read the data in our CSV with both function and class.

Read CSV file with csv.reader

Import csv with open ('example.csv') as a file: reader = csv.reader (File, separator = ",", quotechar = ",", quoting = csv.QUOTE_MINIMAL) for line in the reader: print (row)

In the code above we import the CSV module and then open our CSV file as a. We then define the reader object and use the method to extract the data into the object. We then iterate through the Get Object and get each row of our data.

We display the read data by printing the content on the console. We have also provided the required parameters such as delimiters, quotation marks and quotations.


['first_name', 'last_name', 'Grade'] ['Alex', 'Brian', 'B'] ['Rachael', 'Rodriguez', 'A'] ['Tom', 'smith', 'C ']

Read a CSV file with DictReader

As mentioned earlier, DictWriter makes it possible to read a CSV file by mapping the data to a dictionary instead of strings module. Although the field name is an optional parameter, it is important that your columns are labeled for readability.

How to read a CSV with the DictWriter class.

Import csv results = [] with open ('example.csv') as file: reader = csv.DictReader (file) for line in reader: results.append (row) print results

We first import the csv module and initialize an empty list that we will use to store the retrieved data. We then define the reader object and use the method to extract the data into the object. We then iterate through the Get Object and get each row of our data.

Finally, we append each line to the result list and print the content to the console.


['Grade': 'B', 'first_name': 'Alex', 'last_name': 'Brian', 'grade': 'A', 'first_name': 'Rachael', 'last_name': 'Rodriguez', ' Grade ':' C ',' first_name ':' Tom ',' last_name ':' smith ',' grade ':' B ',' first_name ':' Jane ',' last_name ':' Oscar ',' Grade ' : 'A', 'First name': 'Kennzy', 'Last name': 'Tim']

As you can see above, the DictReader class is better to use because our data is output in a dictionary format that is easier to use.

Write to a CSV file

Now let's see how to go about writing data to a CSV file using the function and class that was discussed at the beginning of this tutorial.

Writing to a CSV file With csv.writer

The following code writes the defined data to the file.

import csv myData = [["first_name", "second_name", "Grade"], ['Alex', 'Brian', 'A'], ['Tom', 'Smith', 'B']] myFile = opened ('example2.csv', 'w') with myFile: writer = csv.writer (myFile) writer.writerows (myData) print ("Writing completed")

First we import the csv module and the function creates an object suitable for writing. In order to iterate the data over the rows we need to use the function.

Here is our CSV with the data we wrote.

Writing to a CSV file with DictWriter

Let's write the following data to a CSV.

data = ['Note': 'B', 'first_name': 'Alex', 'last_name': 'Brian', 'grade': 'A', 'first_name': 'Rachael', 'last_name': 'Rodriguez' , 'Class': 'C', 'First Name': 'Tom', 'Last Name': 'smith', 'Class': 'B', 'First Name': 'Jane', 'Last Name': 'Oscar', 'Grade': 'A', 'First name': 'Kennzy', 'Last name': 'Tim']

The code is as shown below.

Import csv with open ('example4.csv', 'w') as a csv file: fieldnames = ['first_name', 'last_name', 'grade'] writer = csv.DictWriter (csvfile, fieldnames = fieldnames) writer.writeheader () writer.writerow ('grade': 'B', 'first_name': 'Alex', 'last_name': 'Brian') writer.writerow ('grade': 'A', 'first_name': 'Rachael', 'last_name': 'Rodriguez') writer.writerow ('Grade': 'B', 'first_name': 'Jane', 'last_name': 'Oscar') writer.writerow ('Grade': 'B', 'first_name ':' Jane ',' last_name ':' Loive ') print ("Writing completed")

We define that first, the headings of the individual columns are displayed in the CSV file. The method writes in one line at a time. If you want to write all the data at the same time, use the method.

So write on all lines at the same time.

Import csv with open ('example5.csv', 'w') as csvfile: fieldnames = ['first_name', 'last_name', 'grade'] writer = csv.DictWriter (csvfile, fieldnames = fieldnames) writer.writeheader () writer.writerows (['Class': 'B', 'First Name': 'Alex', 'Last Name': 'Brian', 'Class': 'A', 'First Name': 'Rachael', 'Last Name': 'Rodriguez', 'Class':' C ',' First Name ':' Tom ',' Last Name ':' smith ',' Class': 'B', 'First Name': 'Jane', 'last_name': 'Oscar ',' Grade ':' A ',' first_name ':' Kennzy ',' last_name ':' Tim ']) print ("Writing completed")


This tutorial has covered most of what it takes to successfully read and write a CSV file using the various functions and classes provided by Python. CSV files are widely used in software applications because they are easy to read and manage, and because of their small size, they can be processed and transferred relatively quickly.

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