Graphics Reference
In-Depth Information
Dictionaries
Whereas lists are collections of items that are ordered and accessible by an integer index, dictionaries are col-
lections of items that are accessible via reference to other items. Dictionaries are represented by a comma-sep-
arated collection of pairs. The elements of the pairs are separated by colons, and the whole collection is within
curly brackets, as in this example:
{'brit': 32, 'john': 25, 'mary': 29, 'tom': 74}
Each pair is called a key-value pair ; the item to the left of the colon is the key , and the item to the right is
its corresponding value . A value in a dictionary can be accessed by using its key, just as an element of a list is
accessed by using its index:
>>> {'brit': 32, 'john': 25, 'mary': 29, 'tom': 74}['mary']
29
A list of all the keys in a dictionary can be output by using the keys() method, as shown here:
>>> {'brit': 32, 'john': 25, 'mary': 29, 'tom': 74}.keys()
['tom', 'john', 'mary', 'brit']
Operators
Operators are the way a programming language determines the relationships between various data elements in
a program. You've already seen an important operator, the = operator, or assignment operator . The purpose of
this operator is to assign the value on the right of the operator to the variable on the left side of the operator. Be
careful: Although this looks like an equal sign, it is not the sign used to indicate that two terms have the same
value.
Python's comparison operators are shown in Table 12-1 .
Table 12-1: Python's Comparison Operators
Operator Value
Equal to
==
Greater than or equal to
>=
>
Greater than
Less than or equal to
<=
Less than
<
Not equal to
!=
You can see the difference between assignment and equality with this example:
>>> a=5
>>> a
5
>>> a==5
True
>>> a==6
False
Furthermore, logical terms can be put together using either the and or or keyword.
 
 
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