Python Notes (4)
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The note covers two python data types - lists and dictionaries.
Python notes of open courses @Codecademy.
Lists
- Lists
- a datatype you can use to store a collection of different pieces of information as a sequence under a single variable name, e.g,
list_name = [item_1, item_2]
.
- a datatype you can use to store a collection of different pieces of information as a sequence under a single variable name, e.g,
- Access by Index
list_name[index]
- List indices begin with 0, not 1.
- New Neighbors
- A list index behaves like any other variable name! It can be used to access as well as assign values.
list_name[index] = value
- Late Arrivals & List Length:
- A list doesn’t have to have a fixed length.
- You can add items to the end of a list using
list_name.append('sth')
.
- List Slicing
- To access a portion of a list using
list_name[1:2]
. - We start at the index before the colon and continue up to but not including the index after the colon.
- To access a portion of a list using
Slicing Lists and Strings
- Strings can be treated as lists of characters: each character is a sequential item in the list, starting from index 0.
If your list slice includes the very first or last item in a list (or a string), the index for that item doesn’t have to be included. E.g.,
my_list[:2] # Grabs the first two itemsmy_list[3:] # Grabs the fourth through last items
Maintaining Order
list.index(item)
: can find the index of item. The index can be assigned to a variable.list.insert(index, item)
: can insert the item at that index.list.sort()
: can make strings in alphabetical order, or make lists in numerical order.
For One and All
for variable in list_name
:A
variable
name follows thefor
keyword; it will be assigned the value of each list item in turn.# A sample loop would be structured as follows:a = ["List of some sort”]for x in a: # Do something for every x
in list_name
: designateslist_name
as the list the loop will work on. The line ends with a colon:
and the indented code that follows it will be executed once per item in the list.
- Removing Elements from Lists
list.pop(index)
will remove the item atindex
from the list and return it.list.remove(item)
: removes the first item fromlist
that matches theitem
. Note that.remove(item)
does not return anything.del(list[index])
is like.pop
in that it will remove the item at the given index, but it won’t return it.
- Python Range Function
range()
: is just a shortcut for generating a list. It three different versions:range(stop)
range(start, stop)
range(start, stop, step)
- In all cases, the
range()
function returns a list of numbers from start up to (but not including) stop. Each item increases by step. - If omitted, start defaults to zero and step defaults to one.
Iterating Over a List in a Function
Method 1 -
for item in list
:for item in list:print item
- Method 1 is useful to loop through the list, but it’s not possible to modify the list this way.
Method 2 - iterate through indexes:
for i in range(len(list)):print list[i]
- Method 2 uses indexes to loop through the list, making it possible to also modify the list if needed.
Dictionaries
A dictionary is similar to a list, but you access values by looking up a key instead of an index.
- A key can be any string or number.
Dictionaries are enclosed in curly braces, like so:
# Example d = {'key1' : 1, 'key2' : 2, 'key3' : 3}
This is a dictionary called d with three key-value pairs. The key ‘key1’ points to the value 1, ‘key2’ to 2, and so on.
- Dictionaries are great for things like phone books (pairing a name with a phone number), login pages (pairing an e-mail address with a username), and more!
- New Entries
dict_name[new_key] = new_value
- An empty pair of curly braces
{}
is an empty dictionary, just like an empty pair of[]
is an empty list. - The length
len()
of a dictionary is the number of key-value pairs it has. Each pair counts only once, even if the value is a list.
del dict_name[key_name]
: removes items from a dictionary. This will remove the keykey_name
and its associated value from the dictionary.# The key "fish" has a list, the key "cash" has an int, and the key "luck" has a string.my_dict = { "fish": ["c", "a", "r", "p"], "cash": -4483, "luck": "good"}# print "c"print my_dict["fish"][0]
for
loop on a dictionary: to loop through its keys withfor key in dictionary:
- Note that dictionaries are unordered, meaning that any time you loop through a dictionary, you will go through every key, but you are not guaranteed to get them in any particular order.
Tip: The
\
character is a continuation character. The following line is considered a continuation of the current line.
External Resources
- An Introduction to Python Lists
- Dictionaries, Oh Lovely Dictionaries
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