0o25

0o25 is a numeric literal written in octal (base‑8) notation, commonly used in programming languages such as Python and C. The prefix “0o” or “0O” signals that the following digits are interpreted in base eight, and the integer value represented by 0o25 equals 2 × 8 + 5, which is 21 in decimal. In binary, the octal digits correspond to 3‑bit groups, giving 0o25 = 010 101₂.

In C, octal constants were traditionally written with a leading zero (e.g., 025) before the C99 standard required an explicit 0o prefix. Modern C compilers accept both 025 and 0o25, but the leading-zero style is deprecated in newer language revisions. In Python 3, the former style (025) is a syntax error, and the 0o prefix must be used; Python 2 still allowed leading zeros but had the same value semantics.

Octal literals are often employed to express file mode bits in Unix-like systems, where permission settings such as 0755 or 0644 are written in octal. However, 0o25 does not correspond to a common permission triple; it simply represents the decimal number 21, which may appear in small embedded applications, network protocols, or configuration files that use octal for historical reasons.

Because 0o25 can easily be mistaken for the decimal number 25, documentation generally recommends spelling out that the value is octal. The notation is part of a broader family of radix prefixes, including 0b for binary, 0x for hexadecimal, and 0d for decimal in some dialects. The explicit prefix reduces ambiguity and improves code readability, especially when comparing with other numeric bases.