[JonB]

I fancy picking up a pair of pythons as a training shoe since they're reasonably cheap. Quick search here has most people saying drop a full EU size as they stretch a lot. My Miura VS in 41.5 are fairly tight, will a pair of 40.5 Pythons cripple me?

[Muenchener]

If your Miuras are tight, maybe only half size down. I wear Miuras (Laces, not VS, but they seem to size about the same) in 42 for sport, 42.5 for multipitch. Pythons in 41.5. (Street shoe size 45)

[JonB]

Yeah, that seems the sensible option, as my Miura laces are 42 for trad, velcros are 41.5 for bouldering. Thanks!

[blamo]

They aren't lined like the miuras.  So they do stretch a fair amount.  For reference I do 40.5 in Solutions and 39.5s in pythons.  You will likely stretch out the toe box if you go really small.

Beyond being reasonably cheap, they are also really soft so they help strengthen the foot.

Good luck!