TIL #134 – = alignment in string formatting

There are three main alignment options in Python's string formatting:

However, numbers have a fourth option =. On the surface, it looks like it doesn't do anything:

x = 73

print(f"@{x:10}@")   # @        73@
print(f"@{x:=10}@")  # @        73@

But that's because = influences the alignment of the sign. If I make x negative, we already see something:

x = -73

print(f"@{x:10}@")   # @       -73@
print(f"@{x:=10}@")  # @-       73@

So, the equals sign = aligns a number to the right but aligns its sign to the left. That may look weird, but I guess that's useful if you want to pad a number with 0s:

x = -73

print(f"@{x:010}@")  # @-000000073@

In fact, there is a shortcut for this type of alignment, which is to just put a zero immediately to the left of the width when aligning a number:

x = -73

print(f"@{x:010}@")  # @-000000073@

The zero immediately to the left changes the default alignment of numbers to be = instead of >.

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References

• Python 3 Docs, string – Common string operations, Form String Syntax https://docs.python.org/3/library/string.html#format-specification-mini-language [last accessed 04-10-2025];