How to initialise a trainable variable with value of a placeholder in tensorflow V1?

I want to initialise a trainable variable with value of a placeholder in tensorflow V1, tried several times but it seems that the variable does not update itself in the training.

self.a = tf.get_variable("a", initializer=tf.zeros_like(self.a_static))
self.a = tf.assign(self.a, self.a_static)

while self.a_static is a placeholder. In this case self.a seems not updating during training.

When I use

self.a = tf.get_variable("a", initializer=self.a_static)

the program reports error

tensorflow.python.framework.errors_impl.InvalidArgumentError: You must feed a value for placeholder tensor 'a' with dtype float and shape xxx.

but I did feed the placeholder during the training.

Are there any better ways to initialise a trainable variable with placeholder?

Many thanks in advance!