There is nothing quite like a “gotcha” moment in a political interview. On budget day that can mean an unravelling of omnishambles proportions. Last week, the shadow chancellor’s apparent reluctance to answer questions on his own economic policies overshadowed his party’s budget response. Asked repeatedly about the effect of Labour’s economic policies on the nation’s finances, John McDonnell dismissed the questions as “trite” journalism. “That is why we have iPads and … advisers,” he told listeners of BBC Radio 4’s Today programme.

Politicians of all parties have been caught out many times. I remember a schools minister being asked what seven times eight equalled (he was only two out). Such a simple error but it is the only thing I remember from the interview. So is it fair, particularly in an age when we can get instant information for ourselves, to expect politicians to be willing and able to answer every question?

While the shadow chancellor’s frustration may have been understandable, it may not have been advisable

Budget day is always a big day in the political calendar. Delivering a budget is no easy task for a chancellor, but responding to a budget and grabbing some of the attention is also no mean feat for an opposition. The first obstacle is that the government controls the agenda, at least at the start of budget day. They have all the information, preparation and resources. If the economic forecasts are decent and there is money to spend, budget day can feel like wall-to-wall positive coverage for the government. If the economic picture is more difficult – or there is a hidden tax rise or a spending cut that snowballs – the opposition may have more of an opportunity to get into the picture.

The opposition operation has changed little over the years as far as I can gather, with key advisers and shadow ministers shut in the shadow cabinet room waiting for the (short) advance look at the budget documents. Really, the best the opposition can hope for is to communicate its economic message and dividing lines effectively during the day while searching for the holy grail of a pasty tax in the haystack of economic data.

So, if budget day is mainly about the government’s economic policy – either enjoying the spotlight or squirming under the microscope – was McDonnell justified in dismissing questions about Labour’s economic plans? Wednesday’s budget announced a bleak and worrying outlook for our economy. Any shadow chancellor would want the focus to be on those very real economic challenges facing Britain today rather than speculating on the possible effects of future policies of a party not in power.

However, while the shadow chancellor’s frustration may have been understandable, it may not have been advisable. This is not because politicians should be expected to regurgitate every economic fact and figure, it is more because Labour wants to be seen as the government-in-waiting. In more stable political times, at this point in the parliament, the opposition would be analysing why it lost a general election and reconsidering its policy offer in the light of defeat. In these circumstances, it would be fair for an opposition to not have definitive answers on every aspect of its economic plans. But we are not in those times. Labour regularly calls for the Conservatives to move aside – including on Thursday in the shadow chancellor’s speech in the budget debate. Labour cannot expect to have its cake and eat it. If Labour is a government-in-waiting, then it is hardly “trite” journalism to ask how the party’s policies would affect the nation’s finances.

At best McDonnell appeared defensive. At worst, people could think he was not able or not willing to engage.

Sometimes politicians will not have the answers, sometimes they will have forgotten them, sometimes they will never have known them. The fallout can range from slightly embarrassing to a serious questioning of fitness for office, depending on the circumstances. It may not be reasonable to expect our politicians to know every figure and every fact, on the spot; they are only human after all. But when a party is asking for our trust, should it not then expect to be asked for the detail?

In return, our journalism should always prioritise the information we need rather than chase a “gotcha” moment which will get so many more retweets. At the end of the day, an iPad is only as good as the judgment of the person who uses it. The fact that we are still talking about this today shows that the shadow chancellor’s approach distracted attention from Labour’s core argument on the budget. He should have known better.

• Nicola Murphy is a former adviser who has worked in politics, in government and opposition, for over 20 years