Google IT Automation with Python flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Mutable vs immutable types

Lists, dicts, and sets are mutable and change in place, so aliases see edits. Ints, floats, strings, and tuples are immutable; rebinding a name creates a new object instead.

Slice stop exclusion

In s[start:stop:step], the stop index is excluded. s[0:3] returns the first three characters, and a negative step like s[::-1] reverses the sequence.

dict.get default

dict.get(key, default) returns the value if the key exists and the default otherwise, avoiding a KeyError. dict[key] raises KeyError when the key is absent.

try else finally

The else block runs only when the try block raised no exception. The finally block always runs, even after a return, making it suitable for cleanup such as closing files.

with open file mode

open(path, mode, encoding) with a with statement closes the file automatically. Mode r reads, w truncates and writes, a appends, x creates exclusively, and b selects binary mode.

argparse required option

argparse.ArgumentParser().add_argument("--port", type=int, required=True) declares a mandatory integer option. parse_args() returns a namespace whose port attribute is already converted to int.

__main__ guard

Code inside if __name__ == "__main__": executes only when the file is run directly. When the file is imported, __name__ equals the module name and the block is skipped.

Generator expression memory

A generator expression such as (x*x for x in data) yields items lazily and holds one item at a time, while a list comprehension builds the entire list in memory at once.

os.walk tuple output

os.walk(top) yields a tuple (dirpath, dirnames, filenames) for each directory under top. Editing dirnames in place prunes which subdirectories are visited next.

subprocess.run check

subprocess.run(cmd, capture_output=True, text=True, check=True) returns a CompletedProcess with stdout, stderr, and returncode. check=True raises CalledProcessError when the exit code is non-zero.

shutil.copy2 versus move

shutil.copy2 copies a file and preserves metadata such as timestamps. shutil.move relocates or renames a file or directory, removing the source after a successful transfer.

shutil.disk_usage return

shutil.disk_usage(path) returns a named tuple with total, used, and free bytes for the filesystem containing that path. Divide by 1024**3 to express values in gigabytes.

os.scandir DirEntry

os.scandir(path) yields DirEntry objects exposing name, path, is_file(), is_dir(), and stat(). Cached type information avoids a separate system call per directory entry.

os.getpid and getppid

os.getpid() returns the process ID of the running Python process, and os.getppid() returns the ID of its parent. subprocess.Popen exposes the child's PID as the pid attribute.

pathlib glob pattern

pathlib.Path(directory).glob("*.log") yields Path objects matching the pattern in that directory. The rglob method searches recursively through all subdirectories.

os.chmod mode bits

os.chmod(path, 0o755) sets permission bits, where 0o755 grants owner read-write-execute and group and others read-execute. os.stat(path).st_mode reads the current mode.

Git staging area

The staging area, or index, holds changes selected with git add that will enter the next commit. git diff shows unstaged changes; git diff --staged shows staged changes.

Merge versus rebase

git merge joins two histories with a merge commit and preserves both parent lines. git rebase replays commits onto a new base, creating linear history but new commit hashes.

git fetch versus pull

git fetch downloads remote objects and updates remote-tracking branches without touching the working tree. git pull performs a fetch followed by a merge or rebase of the current branch.

git reset hard soft

git reset --soft moves HEAD but keeps changes staged. --mixed also unstages them. --hard additionally discards working-tree changes and is destructive to uncommitted work.

git revert safe undo

git revert creates a new commit that inverts the changes of an earlier commit. It is safe on shared branches because it does not rewrite existing history, unlike git reset.

Detached HEAD state

A detached HEAD occurs when HEAD points directly to a commit instead of a branch, for example after git checkout of a hash. New commits are unreferenced unless a branch is created.

Merge conflict markers

Conflicts insert <<<<<<<, =======, and >>>>>>> markers around competing versions. Edit the file to the intended result, remove markers, run git add, then commit the resolution.

Pull request status checks

GitHub pull requests can require status checks and approving reviews before merging. Protected branches enforce these rules, blocking direct pushes and merges that fail required checks.

git bisect bug search

git bisect performs a binary search over commits between a known good and a known bad revision, marking each tested commit good or bad until the first bad commit is identified.

CSV DictReader usage

csv.DictReader(file) yields one dict per row keyed by the header row, so fields are accessed by name. Open the file with newline="" to avoid blank lines on some platforms.

requests raise_for_status

Response.raise_for_status() raises requests.HTTPError when the status code is 4xx or 5xx. Always pass a timeout argument so a stalled server cannot block the script forever.

unittest setUp tearDown

In unittest.TestCase, setUp runs before each test method and tearDown after it, providing fresh fixtures per test. Class-level variants setUpClass and tearDownClass run once.

pytest fixture decorator

The @pytest.fixture decorator marks a function whose return value is injected into test functions that name it as a parameter. Fixtures handle setup and teardown via yield.

smtplib send_message

smtplib.SMTP(host, port) connects to a mail server; starttls() upgrades to TLS, login() authenticates, and send_message(EmailMessage) sends a message with headers and body.

re.search versus match

re.search scans the whole string for the first match, while re.match only checks at the start. re.findall returns all non-overlapping matches as a list of strings or tuples.

logging.exception in except

Calling logging.exception(message) inside an except block logs the message at ERROR level together with the full traceback, which aids troubleshooting automated scripts that run unattended.