# Importing external files and functions¶

When at all possible, use a Python standard library function instead of writing your own code or downloading a separate package. The standard library functions are available to all Python platforms without any maintenance work from us, they will be more readable to Python programmers (who might just recognize the name instead of having to study the semantics), and those functions are much less likely to contain bugs than our own code is because the library functions have been tested by more people on more tasks.

Non-standard external Python libraries should be used where appropriate, i.e. for anything reasonably substantial like plotting or matrix manipulation. The core external libraries of Topographica are NumPy and PIL (Imaging); you can assume that these will always exist for any Topographica installation. Other external packages, while often very useful, should be considered optional (and their absence should be handled gracefully).

Including an external package adds an approximately fixed cost of tracking future updates and changes to it, handling its licensing issues, increasing the size of our download, restricting the number of supported platforms, etc. Including an external package just for one or two small, simple functions probably doesn’t make sense, but including it for non-trivial items like plot generation or matrix handling does make sense. The key question to answer for any external package is “would the code I’m using from the external package be easier to maintain on its own, or is it easier to just include the external package?”. If it’s easier just to add those couple of functions, just copy them (if the licensing terms allow it); otherwise add the external package.

When importing code, whether from standard libraries or external functions, always import only the functions and classes that you are actually using. As an example that has already occurred, please do NOT do anything like from MLab import *, because of the huge potential for name clashes resulting in strange, hard-to-track bugs. For that particular example, python has a built-in function named max(), but this function is replaced with a matrix-specific version by from MLab import *:

\$ bin/python
>>> max(1.2,0)
1.2
>>> from MLab import *
>>> max(1.2,0)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "/lib/python2.4/site-packages/Numeric/MLab.py", line 146, in max
return maximum.reduce(m,axis)
ValueError: dimension not in array
>>>


Please try to avoid such problems by importing only the specific functions and classes you need, or (where practical) by importing the package only and then using the fully qualified name (e.g. import MLab ; MLab.max()).

If you find that you need to change any file in an external package, please DO NOT change that file, wrap it up into a .tar or .zip archive, and put it back into the Topographica repository. Doing so is a very bad idea, because it prevents us from upgrading that package in the future. Instead, keep the original archive intact as distributed, but then have the external/Makefile patch it automatically to change the specific items that need editing. When the package is updated, the same patch can often be applied as-is to the new package, and in any case it will be clear exactly where to look in the package to make the necessary change.

As a convention, we arrange the imports in each Python file in order from most general to most specific:

1. Python standard library items (such as math, sys, os)

2. Core external packages (numpy and Imaging)

3. Other external packages (such as matplotlib)

4. Topographica files not in the current package

(with absolute paths like topo.base.sheet)

5. Topographica files in the current package

(usually with relative paths like sheet)

A blank line separates each of these sections, and items in the sections are typically ordered alphabetically (at least if there is a long list). For example:

import types
from math import pi

from numpy import transpose, array

from topo.base.connectionfield import CFSheet
from topo.base.functionfamily import TransferFn
from topo.misc.util import flatten, cross_product

from plot import Plot
import bitmap