Python Metrics: Capture Named Arguments

Metrics are one of the most powerful features of Blackfire. They make it possible to enrich your tests within your profiles, and to modify the way a Call-Graph is being rendered, thanks to Argument Capturing.

When a function is being called several times with different arguments, the default behavior is to aggregate these calls into the same node. This behavior can be customized by defining metrics within a .blackfire.yaml file.

Consider the following snippets:

# multipli.py
"""'multipli' module"""

def table(nb, max=10):
	"""Displays the multiplication table of the given number
	from 1 * nb to max * nb"""

	i = 0
	while i < max:
		print(i+1, "*", nb, "=", (i+1) * nb)
		i +=1

# test.py
import multipli

print("Multiplication table - '6'")
print("--------------------------")

multipli.table(nb=6)

print("")

print("Multiplication table - '7'")
print("--------------------------")

multipli.table(nb=7)

print("")

Now let’s profile test.py:

blackfire-python run python test.py

The result is the following Call-Graph:

We can see the 2 calls to the multipli.table function, aggregated into one and only node.

Argument Capturing to the Rescue

It is possible to discriminate the calls by defining a metric and by capturing the arguments. However, in this example, I am using named arguments… The good news is that with the latest versions of the Blackfire Python Probe (>=1.4.3) and of the Agent (>=1.36.0), it is now possible to capture named arguments.

Consider the following metric:

# .blackfire.yaml
metrics:
    multiplication_table:
        label: 'Multiplication Table'
        matching_calls:
            python:
                - callee:
                      selector: "=multipli.table"
                      argument:
                          nb: "*"

Notice the argument section: you can now specify arguments by their names.

The resulting Call-Graph is the following:

One note though: Named-argument capturing only works if the matching functions is being called with named-arguments. If you switch to positional arguments, the function calls will then be aggregated back into one node.

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Happy Profiling!