dfurtado/dataclass-csv dfurtado/dataclass-csv

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Map CSV to Data Classes

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Dataclass CSV

Dataclass CSV makes working with CSV files easier and much better than working with Dicts. It uses Python's Dataclasses to store data of every row on the CSV file and also uses type annotations which enables proper type checking and validation.

Main features

  • Use dataclasses instead of dictionaries to represent the rows in the CSV file.
  • Take advantage of the dataclass properties type annotation. DataclassReader use the type annotation to perform validation of the data of the CSV file.
  • Automatic type conversion. DataclassReader supports str, int, float, complex, datetime and bool, as well as any type whose constructor accepts a string as its single argument.
  • Helps you troubleshoot issues with the data in the CSV file. DataclassReader will show exactly in which line of the CSV file contain errors.
  • Extract only the data you need. It will only parse the properties defined in the dataclass
  • Familiar syntax. The DataclassReader is used almost the same way as the DictReader in the standard library.
  • It uses dataclass features that let you define metadata properties so the data can be parsed exactly the way you want.
  • Make the code cleaner. No more extra loops to convert data to the correct type, perform validation, set default values, the DataclassReader will do all this for you.
  • In additon of the DataclassReader the library also provides a DataclassWriter which enables creating a CSV file using a list of instances of a dataclass.

Installation

pipenv install dataclass-csv

Getting started

Using the DataclassReader

First, add the necessary imports:

from dataclasses import dataclass

from dataclass_csv import DataclassReader

Assuming that we have a CSV file with the contents below:

firstname,email,age
Elsa,[email protected], 11
Astor,[email protected], 7
Edit,[email protected], 3
Ella,[email protected], 2

Let's create a dataclass that will represent a row in the CSV file above:

@dataclass
class User:
    firstname: str
    email: str
    age: int

The dataclass User has 3 properties, firstname and email is of type str and age is of type int.

To load and read the contents of the CSV file we do the same thing as if we would be using the DictReader from the csv module in the Python's standard library. After opening the file we create an instance of the DataclassReader passing two arguments. The first is the file and the second is the dataclass that we wish to use to represent the data of every row of the CSV file. Like so:

with open(filename) as users_csv:
    reader = DataclassReader(users_csv, User)
    for row in reader:
        print(row)

The DataclassReader internally uses the DictReader from the csv module to read the CSV file which means that you can pass the same arguments that you would pass to the DictReader. The complete argument list is shown below:

dataclass_csv.DataclassReader(
    f,
    cls,
    fieldnames=None,
    restkey=None,
    restval=None,
    dialect='excel',
    *args,
    **kwds
)

All keyword arguments support by DictReader are supported by the DataclassReader, with the addition of:

validate_header - The DataclassReader will raise a ValueError if the CSV file cointain columns with the same name. This validation is performed to avoid data being overwritten. To skip this validation set validate_header=False when creating a instance of the DataclassReader, see an example below:

reader = DataclassReader(f, User, validate_header=False)

If you run this code you should see an output like this:

User(firstname='Elsa', email='[email protected]', age=11)
User(firstname='Astor', email='[email protected]', age=7)
User(firstname='Edit', email='[email protected]', age=3)
User(firstname='Ella', email='[email protected]', age=2)

Error handling

One of the advantages of using the DataclassReader is that it makes it easy to detect when the type of data in the CSV file is not what your application's model is expecting. And, the DataclassReader shows errors that will help to identify the rows with problem in your CSV file.

For example, say we change the contents of the CSV file shown in the Getting started section and, modify the age of the user Astor, let's change it to a string value:

Astor, [email protected], test

Remember that in the dataclass User the age property is annotated with int. If we run the code again an exception will be raised with the message below:

dataclass_csv.exceptions.CsvValueError: The field `age` is defined as <class 'int'> but
received a value of type <class 'str'>. [CSV Line number: 3]

Note that apart from telling what the error was, the DataclassReader will also show which line of the CSV file contain the data with errors.

Default values

The DataclassReader also handles properties with default values. Let's modify the dataclass User and add a default value for the field email:

from dataclasses import dataclass


@dataclass
class User:
    firstname: str
    email: str = 'Not specified'
    age: int

And we modify the CSV file and remove the email for the user Astor:

Astor,, 7

If we run the code we should see the output below:

User(firstname='Elsa', email='[email protected]', age=11)
User(firstname='Astor', email='Not specified', age=7)
User(firstname='Edit', email='[email protected]', age=3)
User(firstname='Ella', email='[email protected]', age=2)

Note that now the object for the user Astor have the default value Not specified assigned to the email property.

Default values can also be set using dataclasses.field like so:

from dataclasses import dataclass, field


@dataclass
class User:
    firstname: str
    email: str = field(default='Not specified')
    age: int

Mapping dataclass fields to columns

The mapping between a dataclass property and a column in the CSV file will be done automatically if the names match, however, there are situations that the name of the header for a column is different. We can easily tell the DataclassReader how the mapping should be done using the method map. Assuming that we have a CSV file with the contents below:

First Name,email,age
Elsa,[email protected], 11

Note that now, the column is called First Name and not firstname

And we can use the method map, like so:

reader = DataclassReader(users_csv, User)
reader.map('First name').to('firstname')

Now the DataclassReader will know how to extract the data from the column First Name and add it to the to dataclass property firstname

Supported type annotation

At the moment the DataclassReader support int, str, float, complex, datetime, and bool. When defining a datetime property, it is necessary to use the dateformat decorator, for example:

from dataclasses import dataclass
from datetime import datetime

from dataclass_csv import DataclassReader, dateformat


@dataclass
@dateformat('%Y/%m/%d')
class User:
    name: str
    email: str
    birthday: datetime


if __name__ == '__main__':

    with open('users.csv') as f:
        reader = DataclassReader(f, User)
        for row in reader:
            print(row)

Assuming that the CSV file have the following contents:

name,email,birthday
Edit,[email protected],2018/11/23

The output would look like this:

User(name='Edit', email='[email protected]', birthday=datetime.datetime(2018, 11, 23, 0, 0))

Fields metadata

It is important to note that the dateformat decorator will define the date format that will be used to parse date to all properties in the class. Now there are situations where the data in a CSV file contains two or more columns with date values in different formats. It is possible to set a format specific for every property using the dataclasses.field. Let's say that we now have a CSV file with the following contents:

name,email,birthday, create_date
Edit,[email protected],2018/11/23,2018/11/23 10:43

As you can see the create_date contains time information as well.

The dataclass User can be defined like this:

from dataclasses import dataclass, field
from datetime import datetime

from dataclass_csv import DataclassReader, dateformat


@dataclass
@dateformat('%Y/%m/%d')
class User:
    name: str
    email: str
    birthday: datetime
    create_date: datetime = field(metadata={'dateformat': '%Y/%m/%d %H:%M'})

Note that the format for the birthday field was not speficied using the field metadata. In this case the format specified in the dateformat decorator will be used.

Handling values with empty spaces

When defining a property of type str in the dataclass, the DataclassReader will treat values with only white spaces as invalid. To change this behavior, there is a decorator called @accept_whitespaces. When decorating the class with the @accept_whitespaces all the properties in the class will accept values with only white spaces.

For example:

from dataclass_csv import DataclassReader, accept_whitespaces

@accept_whitespaces
@dataclass
class User:
    name: str
    email: str
    birthday: datetime
    created_at: datetime

If you need a specific field to accept white spaces, you can set the property accept_whitespaces in the field's metadata, like so:

@dataclass
class User:
    name: str
    email: str = field(metadata={'accept_whitespaces': True})
    birthday: datetime
    created_at: datetime

User-defined types

You can use any type for a field as long as its constructor accepts a string:

class SSN:
    def __init__(self, val):
        if re.match(r"\d{9}", val):
            self.val = f"{val[0:3]}-{val[3:5]}-{val[5:9]}"
        elif re.match(r"\d{3}-\d{2}-\d{4}", val):
            self.val = val
        else:
            raise ValueError(f"Invalid SSN: {val!r}")


@dataclasses.dataclass
class User:
    name: str
    ssn: SSN

Using the DataclassWriter

Reading a CSV file using the DataclassReader is great and gives us the type-safety of Python's dataclasses and type annotation, however, there are situations where we would like to use dataclasses for creating CSV files, that's where the DataclassWriter comes in handy.

Using the DataclassWriter is quite simple. Given that we have a dataclass User:

from dataclasses import dataclass


@dataclass
class User:
    firstname: str
    lastname: str
    age: int

And in your program we have a list of users:

users = [
    User(firstname="John", lastname="Smith", age=40),
    User(firstname="Daniel", lastname="Nilsson", age=10),
    User(firstname="Ella", "Fralla", age=4)
]

In order to create a CSV using the DataclassWriter import it from dataclass_csv:

from dataclass_csv import DataclassWriter

Initialize it with the required arguments and call the method write:

with open("users.csv", "w") as f:
    w = DataclassWriter(f, users, User)
    w.write()

That's it! Let's break down the snippet above.

First, we open a file called user.csv for writing. After that, an instance of the DataclassWriter is created. To create a DataclassWriter we need to pass the file, the list of User instances, and lastly, the type, which in this case is User.

The type is required since the writer uses it when trying to figure out the CSV header. By default, it will use the names of the properties defined in the dataclass, in the case of the dataclass User the title of each column will be firstname, lastname and age.

See below the CSV created out of a list of User:

firstname,lastname,age
John,Smith,40
Daniel,Nilsson,10
Ella,Fralla,4

The DataclassWriter also takes a **fmtparams which accepts the same parameters as the csv.writer, for more information see: https://docs.python.org/3/library/csv.html#csv-fmt-params

Now, there are situations where we don't want to write the CSV header. In this case, the method write of the DataclassWriter accepts an extra argument, called skip_header. The default value is False and when set to True it will skip the header.

Modifying the CSV header

As previously mentioned the DataclassWriter uses the names of the properties defined in the dataclass as the CSV header titles, however, depending on your use case it makes sense to change it. The DataclassWriter has a map method just for this purpose.

Using the User dataclass with the properties firstname, lastname and age. The snippet below shows how to change firstname to First name and lastname to Last name:

with open("users.csv", "w") as f:
   w = DataclassWriter(f, users, User)

   # Add mappings for firstname and lastname
   w.map("firstname").to("First name")
   w.map("lastname").to("Last name")

   w.write()

The CSV output of the snippet above will be:

First name,Last name,age
John,Smith,40
Daniel,Nilsson,10
Ella,Fralla,4

Copyright and License

Copyright (c) 2018 Daniel Furtado. Code released under BSD 3-clause license

Credits

This package was created with Cookiecutter and the audreyr/cookiecutter-pypackage project template.

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