Read Attributes and Method Calls

Read dot notation from the object outward, distinguish selecting an attribute from calling a method, and keep dictionary keys separate.

The Chapter 8 report used expressions such as path.name and counts.most_common(3). Both place a name after a dot, but they do different work. Reading library code becomes easier once we separate selecting an attribute from calling a method.

Read the Object Before the Dot

Begin with the path to the measurement file:

measurements.txt

An object is a Python value. Here, path refers to a Path object. An attribute is a named value provided by an object. Read path.name from left to right:

take the object referred to by path;
select its attribute named name;
use the value stored there.

No parentheses follow name, so this expression selects a value without calling it. The attribute contains the final name in the path.

Q1. Read a path attribute

Given path = Path("reports/summary.txt"), which expression obtains the final name "summary.txt"?

Choose one

path.name

path.name()

path["name"]

Select one choice, then check.

HintLook for a value rather than an action

The documentation describes name as an attribute containing the final path component.

SolutionSelect path.name

Use path.name. The expression selects the name attribute and returns the string "summary.txt".

Selection and Calling Are Separate Steps

An attribute can itself be a method. A method is a function selected from an object. Consider a short piece of text from the measurement data:

<class 'builtin_function_or_method'>
['north', 'west', 'café']

text.split selects the method object. It does not split anything. text.split() selects that method and then calls it. The parentheses are the visible difference between keeping an operation and performing it.

Arguments go inside the call parentheses. Chapter 8 counted the accepted sensor labels with a Counter:

[('north', 2), ('west', 2), ('café', 1)]

Read counts.most_common(3) in four parts:

Part	Role
`counts`	the `Counter` object
`.most_common`	select the method
`(3)`	call it with the argument `3`
returned value	a list containing three `(label, count)` tuples

The result is a new list. It is not an attribute stored permanently under the name most_common.

Q2. Separate selection from a call

What value does the final expression produce?

Choose one

["north", "west", "café"]

The selected method object, without running it.

A list containing every individual character.

Select one choice, then check.

HintFollow the parentheses

method refers to the same callable selected by text.split. The final parentheses perform the split.

SolutionThe call returns three words

method() calls the selected split method. With no separator argument, it returns ["north", "west", "café"].

A Key Is Not an Attribute

The parser represented one accepted row as a dictionary in this example:

Square brackets ask the dictionary for the value stored under the key "label". A dot asks an object for an attribute. These requests are not interchangeable:

print(record.label)

AttributeError: 'dict' object has no attribute 'label'

The error is evidence. It names the actual type, dict, and the missing attribute, label. Inspect the object before trying another spelling:

<class 'dict'>
dict_keys(['label', 'reading'])

The type and keys show that record["label"] is the intended access. If the object were a class instance with a label attribute, dot notation could be correct. The object's type determines which names and operations it provides.

Q3. Repair three dot expressions

Repair the marked expressions. Read the path's final name, read the dictionary key, and call most_common so the program prints the three expected lines.

Editable Python

HintMatch syntax to the object

A Path supplies name as an attribute. A dictionary stores the two values under string keys. A method performs work only when called.

SolutionUse an attribute, two keys, and one call

Each expression now matches the kind of access supported by its object.

Read dot notation from the object outward. A dot selects an attribute; parentheses call a selected method; square brackets perform a dictionary-key lookup. When an attribute is missing, use the reported type as evidence before guessing. Documentation can then tell us what arguments a valid call accepts and what value it returns.