How does event.mod work?

It’s a bitmask of modifier flags (Shift, Ctrl, Alt, Caps, Num). Multiple modifiers can be active simultaneously. Test with bitwise AND: if event.mod & pygame.KMOD_SHIFT — not == KMOD_SHIFT, which fails when other modifiers are also held.

What about left vs right Shift?

KMOD_SHIFT covers either; KMOD_LSHIFT and KMOD_RSHIFT are specific. Use KMOD_SHIFT unless you specifically want to distinguish. Same pattern for Ctrl (KMOD_LCTRL/KMOD_RCTRL/KMOD_CTRL).

Should I check mod in KEYDOWN or use get_mods?

For one-shot key presses, check event.mod in KEYDOWN. For continuous state (is Shift currently held), use pygame.key.get_mods() which queries current state.

Fix: Pygame Keyboard Modifier mod Not Captured

Quick answer: Use bitwise AND, not equality: if event.mod & pygame.KMOD_SHIFT. event.mod is a bitmask of all active modifiers.

Shift+S should save the game. Players report it doesn’t work when Caps Lock is on. Your code does if event.mod == pygame.KMOD_SHIFT — which fails because event.mod has the Caps bit set too.

Bitmask vs Equality

Multiple modifiers can be active. event.mod is the OR of every active modifier:

event.mod = KMOD_SHIFT | KMOD_CAPS   # Shift held + Caps on

Comparing with == KMOD_SHIFT requires Shift alone. Use AND to test presence:

if event.type == pygame.KEYDOWN:
    if event.mod & pygame.KMOD_SHIFT and event.key == pygame.K_s:
        save_game()

Tests only the Shift bit; other modifiers can be on or off without affecting the test.

Common Modifiers

KMOD_SHIFT (either Shift), KMOD_LSHIFT, KMOD_RSHIFT
KMOD_CTRL (either Ctrl), KMOD_LCTRL, KMOD_RCTRL
KMOD_ALT (either Alt), KMOD_LALT, KMOD_RALT
KMOD_CAPS (Caps Lock state)
KMOD_NUM (Num Lock state)
KMOD_META (Windows/Command key)

Querying Current State

mods = pygame.key.get_mods()
if mods & pygame.KMOD_SHIFT:
    running()

For continuous checks (is shift currently held while moving), get_mods queries the live state independently of events.

Exclusive Modifier Test

If you specifically need “only Shift, no other modifiers”:

only_shift = (event.mod & ~pygame.KMOD_CAPS & ~pygame.KMOD_NUM) == pygame.KMOD_SHIFT

Strips ignored state (Caps/Num) and tests the remaining for exact Shift. Rare requirement; usually AND is what you want.

Verifying

Test Shift+S with Caps off, with Caps on, with Num on, etc. Save should fire in all cases. Without the fix, Caps-locked save attempts silently fail.

“event.mod is a bitmask. AND for presence. Equality is wrong unless you specifically want exact-state modifiers.”

Treat modifier checks like permission flags: AND-test for ‘has it’, not equality. Same pattern across most input APIs.