CODEWITHSUNDEEP
pythonpython-3.xwarnings
Erel Segal-Halevi
Erel Segal-Halevi

Reputation: 36715

Issue a warning only once per program

I have a function with some user warning:

warnings.warn(message)

I want to make it display the message only at the first time it is encountered, and then become silent.

The closest thing I found in the docs was to issue, at the program start:

warnings.simplefilter('once', UserWarning)

But it does not seem to work - I get the same warning many times in the same program.

How can I do this?

Upvotes: 2

Views: 549

Answers (1)

Patrick Artner
Patrick Artner

Reputation: 51643

You could construct a class that does and memoizes given warnings for you:

class WarnOnlyOnce:
    warnings = set()

    @classmethod
    def warn(cls,message):
        # storing int == less memory then storing raw message
        h = hash(message) 
        if h not in cls.warnings:
            # do your warning
            print(f"Warning: {message}")
            cls.warnings.add(h)


WarnOnlyOnce.warn("First warning")
WarnOnlyOnce.warn("First warning")   # skipped - identical spelling
WarnOnlyOnce.warn("Second warning")
WarnOnlyOnce.warn("First warning")   # skipped - identical spelling
WarnOnlyOnce.warn("First warning")   # skipped - identical spelling
WarnOnlyOnce.warn("Last warning")   
WarnOnlyOnce.warn("LAst warning")    # different spelling
WarnOnlyOnce.warn("LASt warning")    # different spelling
WarnOnlyOnce.warn("LAST warning")    # different spelling

Output:

Warning: First warning
Warning: Second warning
Warning: Last warning
Warning: LAst warning
Warning: LASt warning
Warning: LAST warning

Upvotes: 1

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