Westminster loves a by-election. Opinion polls can tell us only what people say they might do, which isn't even a reliable guide to what they truly think, let alone what will happen when they are ultimately confronted with a ballot paper. A good by-election reveals the mood of the electorate - albeit in just one constituency. It also helps parties test campaigning strategies. Labour tacticians still wince at the memory of the ill-judged deployment of top-hatted stalkers to pursue the (not at all aristocratic) Conservative candidate in Crewe and Nantwich in 2008.
And, of course, the by-election is often, in the worn idiom of political journalism, "a crucial test" for party leaders. Are they showing momentum? Are they fending off attacks? Is their message getting through?
So it is hardly surprising that the prospect of a rash of constituency skirmishes over the next year or so is generating a lot of chatter in parliament. In May of this year there will be referendums in 11 British cities on the creation of an elected mayoral job. If the answer is "yes", mayors will be chosen in November. Also due in November are votes to install the first 40 directly elected local police commissioners. Some of these jobs might be fancied by sitting MPs, especially on the Labour side where life in impotent opposition is proving dispiriting.
Two names often cited as possible candidates for the Birmingham mayoralty are Liam Byrne, shadow work and pensions secretary and MP for the city's Hodge Hill constituency, and Gisela Stuart, MP for Edgbaston. Of the two, fans of politics as spectator sport would much prefer the latter for the simple and faintly dishonorable reason that the battle to fill Stuart's seat would be by far the more exciting. Edgbaston is a bellwether constituency. It was the result - a massive swing to Labour - whose declaration on election night in May 1997 revealed the scale of the landslide to come. If Edgbaston had gone Labour, anything was possible.
Likewise, Labour holding the seat in 2010 was a telling sign that Cameron had failed to fully decontaminate the Tory party. If his campaign had gone anything like according to plan he really ought to have won that seat. The fact that he didn't is sometimes attributed to the personal standing of the local MP, sometimes also to a well-fought grass roots campaign. In any case, it's a marginal now and one that the Tories would very much like to snatch away from Ed Miliband.
Rivalling Edgbaston for interest is the Hampshire seat of Eastleigh, currently occupied by Chris Huhne. The former Lib Dem Energy Secretary faces criminal charges on a driving offence. He is, of course, entitled to be presumed innocent. If, however, he is convicted he would have to quit as an MP. The seat has, in the past, been a straight Lib Dem/Tory contest. A by-election along those lines could be gruesome for coalition relations. Quite a few Lib Dem seats match that profile, including constituencies held by cabinet ministers. Vince Cable, Business Secretary (Twickenham) and Ed Davey, Huhne's replacement as Energy Secretary (Kingston and Surbiton) would both be watching with keen interest how the Tories would campaign in an Eastleigh contest.
All of this is, of course, still very much in the realm of idle speculation and hypothesis. But that doesn't mean it isn't exercising a lot of minds in Westminster.