A peace officer, when making an arrest by virtue of a warrant, shall inform the person to be arrested of his authority and of the fact that a warrant has been issued for his arrest, unless he flees or forcibly resists before the officer has an opportunity to inform him, or unless the giving of such information would imperil the arrest.  The officer need not have the warrant in his possession at the time of the arrest, but after the arrest, if the person arrested so requests, the warrant shall be shown to him as soon as practicable.