Try:
form = MyForm({'charfield1': 'foo', 'charfield2': 'bar'})
The constructor of Form objects can take a dictionary of field values. This creates a bound form, which can be used to validate the data and render the form as HTML with the data displayed. See the forms API documentation for more details.
Edit:
For the sake of completeness, if you do not want to bind the form, and you just want to declare initial values for some fields, you can use the following instead:
form = MyForm(initial={'charfield1': 'foo', 'charfield2': 'bar'})
See the documentation of initial values for details.