TerraformでLambdaをデプロイするとき、ローカルでzipファイルを作成しておくか、あらかじめS3にアップしておく必要があります。

resource "aws_lambda_function" "sample_function" {
  filename         = "lambda/upload/sample_function.zip"
  function_name    = "sample_function"
  role             = "${aws_iam_role.lambda_sample_function.arn}"
  handler          = "lambda_function.lambda_handler"
  source_code_hash = "${base64sha256(file("lambda/sample_function/upload.zip"))}"
  runtime          = "python3.6"

  memory_size = 128
  timeout     = 30
}

手動、もしくはなんらかのツールでzipを作成してもいいのですがArchive Providerを使用すればTerraformがZIPファイルを作成してくれるようになります。
https://www.terraform.io/docs/providers/archive/d/archive_file.html

data "archive_file" "sample_function" {
  type        = "zip"
  source_dir  = "lambda/sample_function"
  output_path = "lambda/upload/sample_function.zip"
}

resource "aws_lambda_function" "sample_function" {
  filename         = "${data.archive_file.sample_function.output_path}"
  function_name    = "sample_function"
  role             = "${aws_iam_role.lambda_sample_function.arn}"
  handler          = "lambda_function.lambda_handler"
  source_code_hash = "${data.archive_file.sample_function.output_base64sha256}"
  runtime          = "python3.6"

  memory_size = 128
  timeout     = 30
}

Lambdaのコードを変更してterraform planを実行するとzipが作成され差分が出るのでapplyでそのままデプロイできます。

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  ~ aws_lambda_function.sample_function
      last_modified:    "2018-04-13T06:00:00.000+0000" => <computed>
      source_code_hash: "C2gsFZgweCbQ88wjZk9+TmH+Beo+/FDPTrkRIn8mEfg=" => "PCJ/UxjA1evwRUUwxD9OI5mXWXWEmfSShCKC8LcaDD/Ak="


Plan: 0 to add, 1 to change, 0 to destroy.

元記事はこちら

「TerraformでLambdaをデプロイするときの小技」