Update: WGBH News has learned that Mayor Marty Walsh signed the plastic bag ban on Friday.

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A day after Boston’s City Council voted unanimously to pass a partial ban on plastic bags, Mayor Marty Walsh remained non-committal as to whether he will sign the ordinance, suggesting the possibility that the ban isn’t quite a slam dunk just yet.

The ordinance passed by the council bans thinner plastic bags more prone to deterioration. It also requires that shopkeepers charge a nickel for every (sturdier) plastic bag, or paper bag with handles, they hand out.  

Speaking to reporters at an annual Christmas tree lighting, Walsh said that he is still “reviewing” the proposal, which has been making its way through the council, in various forms, for about a year.

“I’m reviewing the cost,” Walsh said. “People say ‘It’s only five cents on a bag,’ — yeah, but we’re nickel and diming our seniors … so I’m going to take a good review of it before I sign it."

But asked whether that meant Walsh is considering a veto, the mayor demurred:

“I didn’t say I was vetoing it, I said I’m reviewing it. Let me review it first, and after I review it if we need to look into an adjustment we’ll look into doing an adjustment.”

“Adjustment,” however — unlike ‘veto’ — is not a word defined in the City Charter.

Once passed by the City Council, an ordinance can only be signed in whole or vetoed in whole – or, if the mayor takes no action, the entire ordinance becomes law automatically.

The mayor can, in a veto, submit a formal list of “objections” that must be noted in the public record and which the Council may consider in drafting a new ordinance.  

But those objections impact the council’s ability to override a veto by a two-thirds vote.

The fact that the bill passed the council unanimously would seem to suggest that a veto would stand no chance — but it isn’t a given.

Walsh has fifteen days in which to veto — not much time, but some time in which to maneuver. Council members who have been hearing from upset constituents, meanwhile, could change their minds.

And should Walsh wait long enough, a veto could be taken up by a different body of councilors.

Buried deep within the city charter are various provisions whose collective upshot, in this case, means that should Walsh issue a veto on the last day he can, the veto would fall after the end of Council’s legislative calendar and be moved to next year’s docket — to be taken up by three new Council members.

City Councilor President Michelle Wu said she respected that the mayor is taking time to review the ordinance.

“I think this proposal has been a great example of the different roles of different parts of government,” Wu said.

“It is entirely the mayor’s right and his responsibility to be thoughtful in reviewing proposals that we send up to him,” Wu said.