Add support for precision specifiers

Author: codingjoe

README.rst

@@ -2,7 +2,7 @@
 Django Dynamic Filenames
 ========================
 
-Write advanced filename patterns using the `Format Specification Mini-Language`__.
+Write advanced filename patterns using the `Format String Syntax`__.
 
 __ https://docs.python.org/3/library/string.html#format-string-syntax
 
@@ -90,6 +90,10 @@ Field names
 
         .. warning:: Not all file systems support Base64 file names.
 
+    All type specifiers also support precisions to cut the string,
+    e.g. ``{{uuid:.2base32}}`` would only return the first 2 characters of a
+    Base32 encoded UUID.
+
 Type specifiers
 ~~~~~~~~~~~~~~~
 
@@ -103,10 +107,12 @@ Example:
     from dynamic_names import FilePattern
 
     upload_to_pattern = FilePattern(
-        filename_pattern='{app_name:.25}/{model_name:.30}/{instance.title:slug}{ext}'
+        filename_pattern='{app_name:.25}/{model_name:.30}/{instance.title:.40slug}{ext}'
     )
 
     class FileModel(models.Model):
         title = models.CharField(max_length=100)
         my_file = models.FileField(upload_to=upload_to_pattern)
 
+Slug type specifiers also support precisions to cut the string. In the example
+above the slug of the instance title will be cut at 40 characters.

dynamic_filenames.py

@@ -1,5 +1,6 @@
 import base64
 import os
+import re
 import uuid
 from string import Formatter
 
@@ -10,40 +11,48 @@
 
 
 class SlugFormatter(Formatter):
+    format_spec_pattern = re.compile(r'(\.\d+)?([\d\w]+)?')
 
     def format_field(self, value, format_spec):
-        if format_spec == 'slug':
-            return slugify(value)
+        precision, ftype = self.format_spec_pattern.match(format_spec).groups()
+        if precision:
+            precision = int(precision.lstrip('.'))
+        if ftype == 'slug':
+            return slugify(value)[:precision]
         return super().format_field(value=value, format_spec=format_spec)
 
 
 class ExtendedUUID(uuid.UUID):
+    format_spec_pattern = re.compile(r'(\.\d+)?([\d\w]+)?')
 
     def __format__(self, format_spec):
-        if format_spec == '':
-            return str(self)
-        if format_spec == 's':
-            return str(self)
-        if format_spec == 'i':
-            return str(self.int)
-        if format_spec == 'x':
-            return self.hex.lower()
-        if format_spec == 'X':
-            return self.hex.upper()
-        if format_spec == 'base32':
+        precision, ftype = self.format_spec_pattern.match(format_spec).groups()
+        if precision:
+            precision = int(precision.lstrip('.'))
+        if ftype == '':
+            return str(self)[:precision]
+        if ftype == 's':
+            return str(self)[:precision]
+        if ftype == 'i':
+            return str(self.int)[:precision]
+        if ftype == 'x':
+            return self.hex.lower()[:precision]
+        if ftype == 'X':
+            return self.hex.upper()[:precision]
+        if ftype == 'base32':
             return base64.b32encode(
                 self.bytes
-            ).decode('utf-8').rstrip('=\n')
-        if format_spec == 'base64':
+            ).decode('utf-8').rstrip('=\n')[:precision]
+        if ftype == 'base64':
             return base64.urlsafe_b64encode(
                 self.bytes
-            ).decode('utf-8').rstrip('=\n')
+            ).decode('utf-8').rstrip('=\n')[:precision]
         return super().__format__(format_spec)
 
 
 class FilePattern:
     """
-    Write advanced filename patterns using the Format Specification Mini-Language.
+    Write advanced filename patterns using the Format String Syntax.
 
     Basic example:
 
@@ -52,7 +61,7 @@ class FilePattern:
         from django.db import models
         from dynamic_names import FilePattern
 
-        upload_to_pattern = FilePattern('{app_name:.25}/{model_name:.30}/{uuid:base32}{ext}')
+        upload_to_pattern = FilePattern('{app_name:.25}/{model_name:.30}/{uuid:.30base32}{ext}')
 
         class FileModel(models.Model):
             my_file = models.FileField(upload_to=upload_to_pattern)

setup.cfg

@@ -2,7 +2,7 @@
 name = django-dynamic-filenames
 author = Johannes Hoppe
 author-email = info@johanneshoppe.com
-summary = Write advanced filename patterns using the Format Specification Mini-Language.
+summary = Write advanced filename patterns using the Format String Syntax.
 description-file = README.rst
 description-content-type = text/x-rst; charset=UTF-8
 home-page = https://github.com/codingjoe/django-dynamic-filenames

tests/test_dynamic_filenames.py

@@ -103,6 +103,11 @@ def test_call__slug(self):
             instance=DefaultModel(title='best model'), filename='some_file.txt'
         ) == 'best-model.txt'
 
+    def test_call__slug_precision(self):
+        assert FilePattern(filename_pattern='{instance.title:.4slug}{ext}')(
+            instance=DefaultModel(title='best model'), filename='some_file.txt'
+        ) == 'best.txt'
+
     def test_destruct(self):
         assert FilePattern().deconstruct() == ('dynamic_filenames.FilePattern', [], {})
         assert FilePattern(filename_pattern='{name}{ext}').deconstruct() == (
@@ -158,6 +163,11 @@ def test_uuid__super(self):
             format(guid, 'does not exist')
         assert 'unsupported format string passed to ExtendedUUID.__format__' in str(e)
 
+    def test_precision(self):
+        guid = ExtendedUUID('522d6f3519204b0fb82ae8f558af2749')
+        assert len(format(guid, '.11base64')) == 11
+        assert format(guid, '.11base64') == 'Ui1vNRkgSw-'
+
 
 def test_migrations(db):
     """Integration tests for deconstruct method."""