Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Apr. 30, 2021	Apr. 30, 2020
Deferred tax assets
Accrued payroll	$ 889	$ 2,081
Other accruals & reserves	4,053	3,174
Operating lease liability	1,323	2,235
Deferred revenue	1,258	2,959
Depreciation	1,588	1,365
Net operating losses	73,189	40,242
R&D tax credit	4,778	3,617
Stock based compensation	3,690	2,628
Other	327
Other		(7)
Gross deferred tax assets	91,095	58,294
Valuation allowance	(88,015)	(55,812)
Total deferred tax assets	3,080	2,482
Deferred tax liabilities
Prepaid expenses	(1,883)	(436)
Operating lease right-of-use assets	(1,197)	(2,046)
Total deferred tax liabilities	(3,080)	(2,482)
Net deferred tax assets (liabilities)	$ 0	$ 0