Python - More on Collections and Sequences

Python module Y111

Related technical and longer articles

Python List, Python Tuple, Python Dictionary
User defined sorting - what is a callback?
copying an object - copy the reference

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Articles and tips on this subject		updated
3797	zip in Python Zipping has become synonymouse with compressing data in programming terms; originally the use of the word in that sense came from zipping up a bag so that a whole lot of contents were held together, but these days the compression (pushing everything into the bag) is kind of expected. Python's zip function ...	2012-07-14
3439	Python for loops - applying a temporary second name to the same object If you write a for loop in Python to go through each element of a list, you assign an extra (temporary) name to each member of the list while within the loop. So than any changes made to the named variable will also be changes to the object in the original list. However, that's overshaddowed / overtaken ...	2011-09-14
3348	List slices in Python - 2 and 3 values forms, with an uplifting example Python's lists are indexed collection objects. That means that they're rather like arrays in that you look up elements by their position numbers, and the number start at a fixed point (0 in the case of Python); they're not totally like arrays in that they are not stored at unchanging sequential memory ...	2011-07-06
3150	Python dictionaries - mutable and immutable keys and values Lists, Tuples, and dictionaries are the conventional collection variables in Python - but when you stop to consider it, objects and strings are collections too. All of these structures bundle together other elements (members) in various ways. In first dictionary demonstrations, we usually use strings ...	2011-02-10
2996	Copying - duplicating data, or just adding a name? Perl and Python compared When you copy a list in Perl, you're duplicating the data and you end up with two distinct copies ... but when you copy a list in Python, you're copying the reference so that you end up with two names for the same variable - almost like an alias. So in Perl - with two different copies - you end up with ...	2010-10-30
2920	Sorting - naturally, or into a different order There's a natural sort order for many things - for numbers, it's ascending, for words it's a dictionary order, for names it's by surname. But sometimes you want to sort a list of things different, or there's no provided way within a programming language to apply the natural sort to your type of thing. In ...	2010-08-20
2894	Sorting people by their names Please sort into order: Tom Pearce Bill Brewer Peter Gurney Peter Davy Daniel Whiddon Harry Hawk Uncle Tom Cobbley What would you do? Bill Brewer Uncle Tom Cobbley Peter Davy Peter Gurney Harry Hawk Tom Pearce Daniel Whiddon That's not a standard sort order for strings of text ... and if you've got ...	2010-07-30
899	Python - extend v append on a list In Python, you can extend a list and you can append to it as well. What's the difference? If you append a list to another list, you add the new list as a single extra list to the original, thus makingthe original list just one longer with an item that is itself a list. But if you extend a list ...	2010-06-20
2718	Python - access to variables in the outer scope In Python, variables are local to the block in which they're used unless declares in some other way. And that's good news, because the last thing you want in a substantial script is for data to "leak" between functions as can happen in default-accepting Perl or Lua. But there is an exception ... if ...	2010-04-13
1873	List Comprehensions in Python How do you perform an operation on every member of a list, producing a new list? A List Comprehension in Python is a structure in which a for loop is written within a list's square brackets. Its purpose is to allow the programmer to write short code that's used to transform each element of a source list, ...	2008-11-07
1869	Anonymous functions (lambdas) and map in Python Why do you name variables? So that you can use them again later. But if you don't want to use them more than once, why bother with a name at all? Most programming languages create temporary or anonymous variables within a single line, and if you've programmed almost anything, you'll have used them without ...	2008-11-04
1310	Callbacks - a more complex code sandwich When you write a piece of code, you're normally putting the filling into the sandwich; there's a built-in program in your computer that controls the loading and running of the code thst you've written, and there is a whole library of standard pieces of code that you call to perform the low level operations. ...	2007-08-21
1304	Last elements in a Perl or Python list How do we refer to the elements of a list? By index number, starting at zero and stopping one short of the number of elements in the list. So a 20 element list has element numbers 0 to 19. How can we refer to the last element, then? We could write an expression based on the length of the list - ...	2007-08-16
633	Copying a reference, or cloning If you copy a variable in a program, you end up with a duplicate, right? set second $first ... in Tcl second=$first ... in shell second = first ... in Python, Ruby, C and Java $second = $first ... in Perl and PHP Well - ALMOST right. For sure an assignment copies a variable, but where that variable ...	2006-06-09
386	What is a callback? When you write a program, you're usually providing the filling for a sandwich. At the top level, you have the operating system and the compiler or interpreter for the particular language you're using and this provides you with the mechanism to start and end your program, and for it to share the resources ...	2006-06-05

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Examples from our training material

2dli.py	List - multiple dimensions
coldem	Collections - initial setup and reference
coplist.py	List - copy through assignment
copy_levels	alias, shallow copy and deep copy comparison
cuber	using a callback to map a list
deepcop.py	List - deep copy
dsd	Python equivalent of Schwartzian transform
fortan	A list of lists (a matrix?)
furlist.py	Further methods that run on lists - looking things up
indemo	Sorting a dictionary?
lat.py	copy reference and shallow copy
lirev.py	List - revision
namsort.py	Natural sorting of a list of strings
p162	Sample answer - module exercise
rails_dict	Sorting and subsorting into your own order
sal	Sorting in a non-standard way
sby_sname	Sorting by surname
scenario	Encapsulating changes in a function
shallowcop.py	List - shallow copy
slices	list slices - revision
uip2.py	Counting in a dictionary, and sorting
unamsort.py	Sorting of a list of strings - user routine ignoring case

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To teach Python in Manchester

Background information

Some modules are available for download as a sample of our material or under an Open Training Notes License for free download from http://www.training-notes.co.uk.

Topics covered in this module

Complex Structures.
Assignments, shallow and deep copies.
Further operations on lists.
Sorting a list - natural order and user sorts.

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