Refreshbooks provides a simple synchronous API for manipulating FreshBooks invoices, clients, and other data:

from refreshbooks import api

c = api.OAuthClient(
    'example.freshbooks.com',
    'consumerkey',
    'My Consumer Secret',
    'An existing token',
    'An existing token secret',
    user_agent='Example/1.0'
)

response = c.invoice.create(
    invoice=dict(
        client_id='8',
        lines=[
            api.types.line(
                name='Yard Work',
                unit_cost='10',
                quantity='4'
            )
        ]
    )
)

invoice_response = c.invoice.get(
    invoice_id=response.invoice_id
)

print "New invoice created: #%s (id %s)" % (
    invoice_response.invoice.number,
    invoice_response.invoice.invoice_id
)

invoices_response = c.invoice.list()

print "There are %s pages of invoices." % (
    invoices_response.invoices.attrib['pages'],
)

for invoice in invoices_response.invoices.invoice:
    print "Invoice %s total: %s" % (
        invoice.invoice_id,
        invoice.amount
    )

Consumer keys and secrets can be obtained from FreshBooks. This library does not handle negotiating for an OAuth token+secret pair; see the oauth module or the OAuth specification for details.

This library also supports the older token-based API authorization scheme:

c = api.TokenClient(
    'example.freshbooks.com',
    'My API token',
    user_agent='Example/1.0'
)

# ... as above ...

API methods return lxml.objectify.ObjectifiedDataElement trees, which can be manipulated as Python objects with the same structure as the underlying XML.

References:

• http://developers.freshbooks.com/ - The FreshBooks API
• http://developers.freshbooks.com/api/oauth/ - FreshBooks and OAuth