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The following discussion of a subscription is based on the Python Language Reference -- 6.3.2. Subscriptions .

Description

A subscription selects an item of a sequence ( string , tuple or list ) or mapping ( dictionary ) object, as in the following :

primary "[" expression_list "]"

I discussed strings earlier. I will discuss lists in this and the next module. I will discuss tuples and mappings later.

The primary

According to the Python Language Reference -- 6.3.2. Subscriptions

"The primary must evaluate to an object that supports subscription, e.g. a list or dictionary. User-defined objects can support subscription by defining a __getitem__() method."

For example, in the earlier module titled Itse1359-1070-Strings Part 2 . the primary was a reference to an object of type str where the reference was stored in a variable named aStr , as shown in Listing 1 . A diagram showing the variable and the object was shown in Figure 1 in that module.

Listing 1 . A string as a primary.
aStr = "This is a string" print(aStr[0]) #print T print(aStr[3]) #print s

A clarification

This terminology can be a little confusing. Perhaps the following will help to clarify things.

An ordinal number is an adjective that describes the numerical position of an object, e.g., first,second, third, etc.

Indexing is the process of assigning an ordinal number to each data item contained in some sort of a container.

The index is the value of an ordinal number that has been assigned to an item.

A subscription is the process of selecting an item of a sequence ( string , tuple or list ) or mapping ( dictionary ) object.

The expression_list

The earlier description included the term expression_list .

The Python Language Reference -- 6.3.2. Subscriptions states:

"If the primary is a sequence, the expression_list must evaluate to an integer or a slice..."

An example of an integer expression_list

In the string example in Listing 1 , the integer was 0 in one case and 3 in the other case.

Negative integers

If the (index) value is negative, the length of the sequence is added to it to obtain the actual index used to access the sequence.

Example of a negative index

For example, aSequence[-1] selects the last item of the sequence.

This is illustrated in the script shown in Listing 2 , which prints the last character in a string.

Listing 2 . Example of a negative index.
aStr = "xyz" print(aStr[-1]) # the output is z

The length of the string is 3.

The sum of the length and the negative index is 2.

The character in the string at an index value of 2 is "z".

More negative-integer rules

The value resulting from adding the length of the sequence to the specified index value must be a non-negative integer less than the number of items in thesequence.

Then, the subscription selects the item whose index is that value (counting from zero) .

Good and bad negative integers

This is illustrated by the script in Listing 3 , which shows both valid and invalid negative index values. (The index value of -4 violates the above rule, thus producing an IndexError.)

Listing 3 . Good and bad negative integers.
aString = "xyz" print(aString[-1]) print(aString[-2]) print(aString[-3]) print(aString[-4]) #The output from this script is shown below.z yx Traceback (most recent call last):File "1359-1080-03.py", line 9, in<module>print(aString[-4])IndexError: string index out of range

Questions & Answers

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Source:  OpenStax, Itse 1359 introduction to scripting languages: python. OpenStax CNX. Jan 22, 2016 Download for free at https://legacy.cnx.org/content/col11713/1.32
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