Create new branch before transaction

Previously, the new branch was a result of implicit branch creation in
`fs.put_file()` _within_ the context.

Now, since we do not allow the base branch to be created in situ, which
would be very confusing, we simply create it explicitly before the
transaction start.

We do this for both NEW_BRANCH (not sure what it is used for in the end)
and TRAINING_BRANCH.

docs/tutorials/demo_data_science_project.py

@@ -159,9 +159,10 @@ def _maybe_urlretrieve(url: str, filename: str) -> str:
 """
 
 # %%
-NEW_BRANCH_NAME = "transform-raw-data"
+NEW_BRANCH = lakefs.Branch(REPO_NAME, "transform-raw-data", client=fs.client)
+NEW_BRANCH.create("main")
 
-with fs.transaction(REPO_NAME, NEW_BRANCH_NAME) as tx:
+with fs.transaction(REPO_NAME, NEW_BRANCH) as tx:
     fs.put(outfile, f"{REPO_NAME}/{tx.branch.id}/weather-2010.json")
     tx.commit(message="Add 2010 weather data")
 
@@ -241,7 +242,8 @@ def transform_json_weather_data(filepath):
 """
 
 # %%
-TRAINING_BRANCH = "training"
+TRAINING_BRANCH = lakefs.Branch(REPO_NAME, "training", client=fs.client)
+TRAINING_BRANCH.create("main")
 
 with fs.transaction(REPO_NAME, TRAINING_BRANCH) as tx:
     train.to_csv(f"lakefs://{REPO_NAME}/{tx.branch.id}/train_weather.csv")
@@ -356,7 +358,7 @@ def transform_json_weather_data(filepath):
 # %%
 
 # access the data of the previous commit with a lakefs ref expression, in this case the same as in git.
-previous_commit = repo.ref(f"{TRAINING_BRANCH}~").get_commit()
+previous_commit = repo.ref(f"{TRAINING_BRANCH.id}~").get_commit()
 fixed_commit_id = previous_commit.id
 print(fixed_commit_id)