Django测试套件运行报错：amount字段空串转数字失败

问题场景 #

运行如下极简Django测试用例时触发错误：

from django.test import TestCase
    
class TestSetup(TestCase):   
    def test_setUp(self):
        pdb.set_trace()
        # 此处代码已删除，不影响错误触发

报错信息 #

> Destroying old test database for alias 'default'... Traceback (most
> recent call last):   File
> "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/models/fields/__init__.py",
> line 1957, in get_prep_value
>     return float(value) ValueError: could not convert string to float: ''
> 
> The above exception was the direct cause of the following exception:
> 
> Traceback (most recent call last):   File "manage.py", line 22, in
> <module>
>     main()   File "manage.py", line 18, in main
>     execute_from_command_line(sys.argv)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/core/management/__init__.py",
> line 442, in execute_from_command_line
>     utility.execute()   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/core/management/__init__.py",
> line 436, in execute
>     self.fetch_command(subcommand).run_from_argv(self.argv)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/core/management/commands/test.py",
> line 24, in run_from_argv
>     super().run_from_argv(argv)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/core/management/base.py",
> line 412, in run_from_argv
>     self.execute(*args, **cmd_options)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/core/management/base.py",
> line 458, in execute
>     output = self.handle(*args, **options)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/core/management/commands/test.py",
> line 68, in handle
>     failures = test_runner.run_tests(test_labels)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/test/runner.py",
> line 1054, in run_tests
>     old_config = self.setup_databases(   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/test/runner.py",
> line 950, in setup_databases
>     return _setup_databases(   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/test/utils.py",
> line 221, in setup_databases
>     connection.creation.create_test_db(   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/backends/base/creation.py",
> line 78, in create_test_db
>     call_command(   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/core/management/__init__.py",
> line 194, in call_command
>     return command.execute(*args, **defaults)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/core/management/base.py",
> line 458, in execute
>     output = self.handle(*args, **options)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/core/management/base.py",
> line 106, in wrapper
>     res = handle_func(*args, **kwargs)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/core/management/commands/migrate.py",
> line 356, in handle
>     post_migrate_state = executor.migrate(   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/migrations/executor.py",
> line 135, in migrate
>     state = self._migrate_all_forwards(   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/migrations/executor.py",
> line 167, in _migrate_all_forwards
>     state = self.apply_migration(   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/migrations/executor.py",
> line 252, in apply_migration
>     state = migration.apply(state, schema_editor)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/migrations/migration.py",
> line 132, in apply
>     operation.database_forwards(   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/migrations/operations/fields.py",
> line 235, in database_forwards
>     schema_editor.alter_field(from_model, from_field, to_field)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/backends/base/schema.py",
> line 830, in alter_field
>     self._alter_field(   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/backends/postgresql/schema.py",
> line 287, in _alter_field
>     super()._alter_field(   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/backends/base/schema.py",
> line 1025, in _alter_field
>     new_default = self.effective_default(new_field)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/backends/base/schema.py",
> line 429, in effective_default
>     return field.get_db_prep_save(self._effective_default(field), self.connection)   File
> "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/models/fields/__init__.py",
> line 954, in get_db_prep_save
>     return self.get_db_prep_value(value, connection=connection, prepared=False)   File
> "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/models/fields/__init__.py",
> line 947, in get_db_prep_value
>     value = self.get_prep_value(value)   File "/var/www/ptech/venv/lib/python3.8/site-packages/django/db/models/fields/__init__.py",
> line 1959, in get_prep_value
>     raise e.__class__( ValueError: Field 'amount' expected a number but got ''.

相关代码与迁移记录 #

当前Appointment模型定义 #

# models.py
class Appointment(models.Model):
    amount = models.FloatField(blank=True, default=0.0)

历史迁移记录 #

• 曾将amount字段默认值设为空字符串：

# Generated by Django 4.1.3 on 2023-01-10 22:58
from django.db import migrations, models   

class Migration(migrations.Migration):
    dependencies = [
        ('giraffe', '0014_appointment_notes'),
    ]
    operations = [
        migrations.AlterField(
            model_name='appointment',
            name='amount',
            field=models.FloatField(blank=True, default=''),
        ),
    ]

• 移除默认值：

class Migration(migrations.Migration):
    dependencies = [
        ('giraffe', '0015_alter_appointment_amount'),
    ]
    operations = [
        migrations.AlterField(
            model_name='appointment',
            name='amount',
            field=models.FloatField(blank=True),
        ),
    ]

• 重新设置默认值为0.0：

class Migration(migrations.Migration):
    dependencies = [
        ('giraffe', '0016_alter_appointment_amount'),
    ]
    operations = [
        migrations.AlterField(
            model_name='appointment',
            name='amount',
            field=models.FloatField(blank=True, default=0.0),
        ),
    ]

已删除模型的迁移痕迹 #

还存在已删除的UnreimbursedMileage模型的迁移记录，该模型也包含amount字段，测试数据库中可能残留相关数据：

...migrations.CreateModel(
            name='UnreimbursedMileage',
            fields=[
                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('mileage_rate_category', models.CharField(choices=[('FB22a', 'Federal Business Rate, Qtr. 1&2 2022'), ('FB22b', 'Federal Business Rate, Qtr. 3&4, 2022'), ('FB23', 'Federal Business Rate 2023')], default='FB22a', max_length=5)),
                ('num_miles', models.FloatField(blank=True, default=0)),
                ('amount', models.FloatField(blank=True, null=True)),
                ('for_appointment', models.OneToOneField(editable=False, on_delete=django.db.models.deletion.CASCADE, to='giraffe.appointment')),
            ],
            options={
                'verbose_name_plural': 'Unreimbursed Mileage',
            },
        ),

class Migration(migrations.Migration):
    dependencies = [
        ('giraffe', '0054_rename_to_do_next_time_todonexttime_and_more'),
    ]
    operations = [
        migrations.RemoveField(
            model_name='unreimbursedmileage',
            name='for_appointment',
        ),
        migrations.DeleteModel(
            name='Dummy',
        ),
        migrations.DeleteModel(
            name='UnreimbursedMileage',
        ),
    ]

解决方案 #

核心原因 #

测试数据库执行迁移时，中间某一步将amount字段的默认值设为空字符串，后续迁移虽然修正了默认值，但PostgreSQL数据库中可能仍残留空字符串的默认值配置，导致Django尝试将空字符串转换为float时失败。

具体修复步骤 #

1. 彻底清理测试数据库：
   执行命令删除测试数据库（确保测试环境数据可丢弃）：

   dropdb your_test_db_name
   

   或者通过Django命令强制重新创建：

   python manage.py test --keepdb=False
   

2. 修正迁移文件中的错误默认值：
   修改历史迁移文件中设置default=''的那一步，将默认值改为0.0，然后执行以下命令：

   python manage.py migrate giraffe --fake 0015  # 伪造执行到错误迁移的前一步
   python manage.py migrate giraffe
   

3. 清理已删除模型的残留数据：
   手动登录PostgreSQL测试数据库，执行SQL删除残留表：

   DROP TABLE IF EXISTS giraffe_unreimbursedmileage;
   

4. 临时绕过测试数据库迁移问题：
   在settings.py中添加配置，测试时使用内存SQLite数据库：

   import sys
   if 'test' in sys.argv:
       DATABASES['default'] = {
           'ENGINE': 'django.db.backends.sqlite3',
           'NAME': ':memory:'
       }
   

内容的提问来源于stack exchange，提问作者Ron