[baseline] Usage

One line Strings

To create a baseline string that contains a special update mechanism, use triple quotes around the string and instantiate Baseline with it. The resulting Python string object supports natural equality comparisons:

hello.py

from baseline import Baseline

expected = Baseline("""Hello""")

test_string = "Hello World!"

assert test_string == expected

Run the script and observe that the assert raises an exception since the strings are not equal. Because the comparison failed, the tool located the triple quoted baseline string in the source file and updated it with the miscompared value. When the interpretter exited, the tool saved the updated source file using the file extension .py.update):

hello.py.update

expected = Baseline("""Hello World!""")

test_string = "Hello World!"

assert test_string == expected

After reviewing the change with your favorite file differencing tool, accept the change by either manually overwriting the original file or use the baseline command line interface to scan the directory for baseline update files:

$ python -m baseline *
Found baseline updates for:
  hello.py

Hit [ENTER] to update, [Ctrl-C] to cancel

Pressing Enter causes the tool to overwrite the scripts with the new baseline updates and remove the temporary .py.update files.

Multi-Line Strings

The triple quote usage in the Baseline instantiation provides a consistent search and replace mechanism that supports embedding quotation marks and newlines within a baselined string. Embedding newlines improves the strings human readability which makes reviewing updates easier.

For multiline baselined string format, start the string on the line following the opening triple quote delimiter. Insert a line after the baselined string content to hold the closing triple quote delimiter. Indent the closing triple quote delimiter to the indentation level of the baselined string:

from baseline import Baseline

expected = Baseline("""
    THE QUICK BROWN FOX
        JUMPS
    OVER THE LAZY DOG.
    """)

test_string = "THE QUICK BROWN FOX\n    JUMPS\nOVER THE LAZY DOG."

assert test_string == expected

The example above executes without an assertion because the tool strips the leading indentation of every line in the baselined string based on the indentation of the closing triple quote.

Transforms

Often strings to test against a baseline contain substrings that may vary from one execution to the next. Before the comparison, normalize the string by substituting a representative constant value. For example, use a regular expression to transform a variable time into a constant value:

import re
import time

from baseline import Baseline


expected = Baseline("""The time is HH:MM:SS.""")

test_string = "The time is {}.".format(time.strftime("%H:%M:%S"))

assert re.sub(r'\d\d:\d\d:\d\d', 'HH:MM:SS', test_string) == expected

If this is a common operation or there are multiple transformations needed, override the TRANSFORMS class attribute and list the operations to be performed. The tool performs each of the operations on the test string before every comparison.

import re
import time

from baseline import Baseline


def normalize_time(s):
    return re.sub(r'\d\d:\d\d:\d\d', 'HH:MM:SS', s)


class NormalizedBaseline(Baseline):

    """Normalized string baseline."""

    TRANSFORMS = [normalize_time]


expected = NormalizedBaseline("""The time is HH:MM:SS.""")

test_string = "The time is {}.".format(time.strftime("%H:%M:%S"))

assert test_string == expected

Tips and Tricks

Quick Tips

• Take your time and be diligent in your review of baseline updates. Similar to Python itself, this tool provides a lot of rope, don’t hang yourself.
• Put comments above the baseline to provide information to a future maintainer of the important aspects of the baseline that are the focus of the test.
• Feel free to baseline strings with any style triple quotes embedded. The tool adjusts and uses the alternative style. If the test string contains both styles, transform one style into something else before comparison.
• To archive resulting test script updates from a regression test run within a continuous integration system, use the --movepath command line option to move updated scripts to a new location instead of overwriting the original script. T

Initial Baseline Value

To avoid the work of anticipating the exact content of the string baseline, specify an empty baseline in multi-line format and set the indentation level with the closing triple quote:

from baseline import Baseline

expected = Baseline("""
    """)

test_string = "THE QUICK BROWN FOX\n  JUMPS\nOVER THE LAZY DOG."

assert test_string == expected

Run the script and let the tool fill in the string baseline. Then carefully review the baseline update and accept.