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spwd Module Complexity¶
The spwd module reads the Unix shadow password database, which holds each account's password
hash and password-ageing fields. Both functions go through the C library, so the host's
name-service configuration, such as /etc/nsswitch.conf, decides where the entries come from;
with the usual files backend that is /etc/shadow, which normally only root can read.
Removed in Python 3.13
Deprecated in Python 3.11 and removed in Python 3.13 by PEP 594. The page covers the module
as it is on Python 3.11 and 3.12 on Unix; the examples also need privileges to read the
database and an entry for root in it.
n is the entries in the database. The bounds are those of the files backend, and treat one
entry as short - a line of /etc/shadow - so reading or decoding it is O(1). Another backend sets
its own lookup cost.
Complexity Reference¶
Functions¶
| Operation | Time | Space | Notes |
|---|---|---|---|
spwd.getspnam(name) |
O(n) | O(1) | Reads the database in order until the name matches. Raises KeyError when no entry matches, or an OSError such as PermissionError when a backend reports a failure; which one an unprivileged process gets depends on the backend |
spwd.getspall() |
O(n) | O(n) | A list of the entries the backends enumerate. Without privileges that may be fewer than getspnam() finds, or none, rather than an error |
struct_spwd¶
| Operation | Time | Space | Notes |
|---|---|---|---|
spwd.struct_spwd |
O(1) | O(1) | The entry type: a 9-item tuple whose items are also attributes |
struct_spwd.sp_namp |
O(1) | O(1) | Index 0: login name |
struct_spwd.sp_pwdp |
O(1) | O(1) | Index 1: password hash |
struct_spwd.sp_lstchg |
O(1) | O(1) | Index 2: date of the last password change, in days since 1970-01-01 |
struct_spwd.sp_min |
O(1) | O(1) | Index 3: minimum days between changes |
struct_spwd.sp_max |
O(1) | O(1) | Index 4: maximum days between changes |
struct_spwd.sp_warn |
O(1) | O(1) | Index 5: days before the password expires to warn the user |
struct_spwd.sp_inact |
O(1) | O(1) | Index 6: days after the password expires until the account is disabled |
struct_spwd.sp_expire |
O(1) | O(1) | Index 7: date the account expires, in days since 1970-01-01 |
struct_spwd.sp_flag |
O(1) | O(1) | Index 8: reserved |
Looking Up One Account¶
getspnam() reads the database until it finds the name, so one lookup costs up to the whole
database. The entry it returns is a tuple, read by index or by attribute name.
import spwd
entry = spwd.getspnam('root') # O(n)
assert entry.sp_namp == 'root' # O(1)
assert entry[0] == entry.sp_namp # O(1) - index 0 is the login name
assert len(entry) == 9
try:
spwd.getspnam('no such user')
except KeyError:
pass
else:
raise AssertionError('a missing name returned an entry')
Reading Every Account¶
getspall() reads the database once and keeps every entry. For more than a few names, that and a
dict beat a getspnam() call per name, each of which reads the database again.
import spwd
entries = spwd.getspall() # O(n) time and space
by_name = {entry.sp_namp: entry for entry in entries} # O(n)
wanted = ['root', 'no such user']
found = {name: by_name[name] for name in wanted if name in by_name} # O(1) per name
assert found['root'] == spwd.getspnam('root')
assert 'no such user' not in found
Performance Best Practices¶
✅ Do:
- Call
getspnam()for one or a few names - Read the database once with
getspall()and look names up in adictwhen you need many - Use
pwdfor names, user IDs and home directories; it needs no privileges and outlives this module
❌ Avoid:
- Calling
getspnam()in a loop over many names - each call reads the database again - Reading an empty
getspall()as an empty database - without privileges it may return nothing rather than raise
Version Notes¶
- Python 3.11+:
import spwdemits aDeprecationWarning - Python 3.13+: The module is removed;
import spwdraisesModuleNotFoundError