Monthly Archives: April 2010

Split a file on any character in Python

I need to split a big text file on a certain character. I expect I am being thick about this, but [`split`][split] doesn’t quite do what I want because it includes the matching line, whereas I want to split right on the matching character.

My Python answer:

def readlines(filename, endings, chunksize=4096):
“””Returns a generator that splits on lines in a file with the given
line = ”
while True:
buf =
if not buf:
yield line

line = line + buf

while endings in line:
idx = line.index(endings) + len(endings)
yield line[:idx]
line = line[idx:]

if __name__ == “__main__”:
import sys, os

FORMFEED = chr(12) # ASCII 12
basename = os.path.basename(sys.argv[1])
for num, data in enumerate(readlines(open(sys.argv[1]), endings=FORMFEED)):
filename = basename + ‘-‘ + str(num)
open(filename, ‘wb’).write(data)

This is also useful when reading data exported from some old-fashioned Mac application like [Filemaker 5][filemaker] where the line-endings are ASCII 13 not ASCII 10.

This post was inspired by [Lotus Notes][lotus] version 8.5, which is so advanced that to save a message in a file on disk you have to export it as structured text. And if you want to save a whole bunch of messages as individual files you must forget that [drag-and-drop was introduced with System 7][mactech], that would be too obvious.


Django AdminForm objects and templates

I can’t find documentation for the context of a Django admin template. In particular, where is the form and how does one access the fields? This post describes the template context for a generic admin model for [Django 1.1][django11].

Django uses an instance of `ModelAdmin` (defined in [`django.contrib.admin.options`][options]) to handle the request for a model object add / change view in the admin site. `ModelAdmin.add_view` and `ModelAdmin.change_view` are responsible for populating the template context when rendering the add object and change object pages respectively.

Here are the keys common to add and change views:

– **title**, ‘Add ‘ or ‘Change ‘ + your model class’ `_meta.verbose_name`
– **adminform** is an instance of `AdminForm`
– **is_popup**, a boolean which is true when `_popup` is passed as a request parameter
– **media** is an instance of [`django.forms.Media`][media]
– **inline_admin_formsets** is a list of [`InlineAdminFormSet`][inlineset] objects
– **errors** is an instance of [`AdminErrorList`][errors]
– **root_path** is the `root_path` attribute of the `AdminSite` object
– **app_label** is your model class’ `_meta.app_label` attribute

The way that Django renders a form in the admin view is to iterate over the `adminform` instance and then iterate over each [`FieldSet`][fieldset] which in turn yield [`AdminField`][adminfield] instances. All I want to do is layout the form fields, ignoring the fieldset groupings which may or may not be defined in the model’s `ModelAdmin.fieldset` attribute.

This turns out to be easy once you know how. The regular form is an attribute of the `adminform` object. So if your model has a field named “`king_of_pop`” you can refer to the form field in your template like so:

{{ adminform.form.king_of_pop.label_tag }}: {{ adminform.form.king_of_pop }}

Or if you want to save your finger tips you can use the [`with` template tag][with]:

{% with adminform.form as f %}
{{ f.king_of_pop.label_tag }}: {{ f.king_of_pop }}
{% endwith %}

Delving through the Django source while I tried to understand all of this I was struck by how [Python defines hook functions for iteration and accessing attributes][hooks]. Half of Python’s attraction is in how easy it is from the program author’s point of view to treat objects as built-in types like lists, dicts, etc.; the other half is the responsibility of the author of a Python module to encourage that same ease of use by implementing the related iteration protocols. It is harder to write a good Python module than it is to write a good Python program that uses a good module.