Key Concepts

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Class csv.DictWriter

Class csv.DictWriter

# An example of csv.DictWriter import csv with open('companies.csv', 'w') as csvfile: fieldnames = ['name', 'type'] writer = csv.DictWriter(csvfile, fieldnames=fieldnames) writer.writeheader() writer.writerow({'name': 'Codecademy', 'type': 'Learning'}) writer.writerow({'name': 'Google', 'type': 'Search'}) """ After running the above code, companies.csv will contain the following information: name,type Codecademy,Learning Google,Search """

In Python, the csv module implements classes to read and write tabular data in CSV format. It has a class DictWriter which operates like a regular writer but maps a dictionary onto output rows. The keys of the dictionary are column names while values are actual data.

The csv.DictWriter constructor takes two arguments. The first is the open file handler that the CSV is being written to. The second named parameter, fieldnames, is a list of field names that the CSV is going to handle.

Learn Python: Files
Lesson 1 of 1
  1. 1
    Computers use file systems to store and retrieve data. Each file is an individual container of related information. If you’ve ever saved a document, downloaded a song, or even sent an email you’ve …
  2. 2
    When we read a file, we might want to grab the whole document in a single string, like .read() would return. But what if we wanted to store each line in a variable? We can use the .readlines() func…
  3. 3
    Sometimes you don’t want to iterate through a whole file. For that, there’s a different file method, .readline(), which will only read a single line at a time. If the entire document is read line b…
  4. 4
    Reading a file is all well and good, but what if we want to create a file of our own? With Python we can do just that. It turns out that our open() function that we’re using to open a file to read …
  5. 5
    So maybe completely deleting and overwriting existing files is something that bothers you. Isn’t there a way to just add a line to a file without completely deleting it? Of course there is! Instead…
  6. 6
    We’ve been opening these files with this with block so far, but it seems a little weird that we can only use our file variable in the indented block. Why is that? The with keyword invokes something…
  7. 7
    Text files aren’t the only thing that Python can read, but they’re the only thing that we don’t need any additional parsing library to understand. CSV files are an example of a text file that impos…
  8. 8
    Recall our CSV file from our last exercise: users.csv Name,Username,Email Roger Smith,rsmith,[email protected] Michelle Beck,mlbeck,[email protected] Ashley Barker,a_bark_x,[email protected]…
  9. 9
    I need to level with you, I’ve been lying to you for the past two exercises. Well, kind of. We’ve been acting like CSV files are Comma-Separated Values files. It’s true that CSV stands for that, bu…
  10. 10
    Naturally if we have the ability to read different CSV files we might want to be able to programmatically create CSV files that save output and data that someone could load into their spreadsheet s…
  11. 11
    CSV isn’t the only file format that Python has a built-in library for. We can also use Python’s file tools to read and write JSON. JSON, an abbreviation of JavaScript Object Notation, is a file for…
  12. 12
    Naturally we can use the json library to translate Python objects to JSON as well. This is especially useful in instances where you’re using a Python library to serve web pages, you would also be a…
  13. 13
    Now you know all about files! You were able to: - Open up file objects using open() and with. - Read a file’s full contents using Python’s .read() method. - Read a file line-by-line using .read…

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