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Define Views for Junos PyEZ Operational Tables that Parse Structured Output

Junos PyEZ operational (op) Tables for structured output select specific data from the XML output of an RPC executed on a Junos device. A Table is associated with a View, which is used to access fields in the Table items and map them to user-defined Python variables. You associate a Table with a particular View by including the view property in the Table definition, which takes the View name as its argument.

A View maps your user-defined variables to XML elements in the selected Table items. A View enables you to access specific fields in the output as variables with properties that can be manipulated in Python. Junos PyEZ handles the extraction of the data into Python as well as any type conversion or data normalization. The keys defined in the View must be valid Python variable names.

Junos PyEZ Views, like Tables, are formatted using YAML. Views that parse structured output can include a number of parameters, which are summarized in Table 1.

Table 1: Parameters in Views for Junos PyEZ Op Tables for Structured Output

View Parameter Name

View Parameter

Description

View name

–

User-defined identifier for the View.

Field items

fields

Associative array, or dictionary, of key-value pairs that map user-defined field names to XPath expressions that select elements in the Table items. The field names must be valid Python variable names.

Field groups

fields_group

Associative array, or dictionary, of key-value pairs that map user-defined field names to XPath expressions that select elements in the Table items. The XPath expressions are relative to the context set by the corresponding groups parameter. The field names must be valid Python variable names.

Groups

groups

Associative array, or dictionary, of key-value pairs that map a user-defined group name to an XPath expression (relative to the Table item context) that sets the XPath context for fields in that group.

Consider the following Junos PyEZ op Table and View, which are included with the Junos PyEZ distribution. The Table extracts operational state information for Ethernet interfaces on the target device.

The following sections discuss the different components of the View:

View Name

The View name is a user-defined identifier for the View. You associate a Table with a particular View by including the view property in the Table definition and providing the View name as its argument. For example:

Fields (fields)

You customize Views so that they only reference the necessary elements from the selected Table items. To do this you include the fields property and an associative array containing the mapping of user-defined field names to XPath expressions that select the desired elements from the Table item. The field names must be valid Python variable names. The XPath expressions are relative to the Table item context.

Consider the following sample RPC output:

If the Table item parameter selects <physical-interface> elements from the output, the XPath expression for each field in the View definition is relative to that context. The following View definition maps each user-defined field name to a child element of the <physical-interface> element:

In the Python script, you can then access a View item as a variable property. By default, each View item has a name property that references the key that uniquely identifies that item.

The field format determines the type for a field’s value. By default, field values are stored as strings. You can specify a different type for the field value in the field mapping. The following example defines the value of the mtu element to be an integer:

In the RPC output, some Junos XML elements are just empty elements that act as flags. You can explicitly indicate that a field is a flag in the field mapping. The field item value for a flag is True if the element is present in the output and False if the element is absent. The following example defines the ifdf-running element as a flag:

You can also set the field item value to a Boolean by using the following syntax:

The element’s value is evaluated against the regular expression passed to regex(). If the element’s value matches the expression, the field item value is set to the Boolean defined in the format. In the following example, the oper_status_down field is set to True if the value of the oper-status element contains 'down':

You can also use more complex regular expressions and match against multiple values. The following field item is set to True if the address in the rt-destination element starts with '198.51.':

The following field item is set to True if the no-refresh element contains either value in the regular expression.

Groups (groups) and Field Groups (fields_)

Groups provide a shortcut method to select and reference elements within a specific node-set in a Table item.

In the following RPC output, the <if-device-flags> element contains multiple child elements corresponding to values displayed in the Device flags field in the CLI output:

Within the View definition, you can use the fields property to access the child elements by providing the full XPath expression to each element relative to the selected Table item. For example, if the EthPortTable definition selects <physical-interface> items, the field item mapping would use the following XPath expressions:

Alternatively, you can create a group that sets the context to the <if-device-flags> element and then define field group items that just provide the XPath expression relative to that context. You can define any number of groups within a View definition.

To create a group, include the groups property and map a user-defined group name to the XPath expression that defines the new context. Then define a field group whose name is fields_ followed by the group name. The field group is an associative array containing the mapping of user-defined field names to XPath expressions that now are relative to the context set within groups. The field names must be valid Python variable names.

The following example defines the group flags and the corresponding field group fields_flags. The flags group sets the context to the physical-interface/if-device-flags hierarchy level, and the present and running fields access the values of the ifdf-present and ifdf-running elements, respectively.

Whether you use fields or field groups, you access the value in the same manner within the Junos PyEZ script by using the user-defined field names.

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